More recently, Bagwell and Staiger have argued that the MFN externality can be suppressed by reciprocity, defined as bilateral liberalization (subject to MFN) aimed at holding constant world relative prices. To see this point, consider the pattern of trade depicted in figure 1(a), involving three countries, A, B and C, and two goods, 1, and 2. If country A lowers its tariff on imports of good 1 on an MFN basis, it improves the terms of trade of both B and C. If B cuts its tariff on good 2 in exchange, it worsens the terms of trade of C, thereby mitigating the MFN externality.

这是一篇论文里的一段，可是后面关于三个国家的部分，怎么也想不明白为什么B国降了关税C国的贸易状况就会恶化。

论文还提供了一张图，是开始时BC向A国出口1，之后A国向BC出口2。为什么不是C和A出口呢？