data a;                                                                                                                                
input t A B;                                                                                                                           
cards;                                                                                                                                 
2006 1 2                                                                                                                               
2005 2 3                                                                                                                               
2004 3 4                                                                                                                               
1998 5 6                                                                                                                               
1997 6 7                                                                                                                               
1996 7 8                                                                                                                               
;                                                                                                                                      
run;                                                                                                                                   
                                                                                                                                       
                                                                                                                                       
proc sort data=a;                                                                                                                      
 by t;                                                                                                                                 
run;                                                                                                                                   
                                                                                                                                       
                                                                                                                                       
data b (keep=t C) ;                                                                                                                    
        set a;                                                                                                                         
        lag_B=lag(B);                                                                                                                  
 if t=lag(t)+1 then do C=A/lag_B                                                                                                       
 ;end;                                                                                                                                 
run;                                                                                                                                   
                                                                                                                                       
proc print data=b;                                                                                                                     
run;                                                                                                                                   
     

lili83xu's solution is perfect! What I added is nothing new - just to make sure C is set to missing when it is supposed to be so.

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data b(keep=t C);
set a;
lag_B=lag(B);
if t=lag(t)+1 then C=A/lag_B;
else C=.;
run;