Assume the demand for watermelons is downward sloping. An increase in price from $3 per pound to $4 per pound

[A] could have been caused by a decrease in supply.

[B] will cause an increase in supply.

[C] will cause a greater quantity of watermelons to be demanded.

[D] will cause demand to decrease.

[E] will cause supply  to decrease.

我不知道该选A或c或d