请教几道 逻辑题

insect infestations in certain cotton-growing regions of the world have caused dramatic increases in the price of cotton on the world market.By contrast,the price of soybeans has long remained stable.Knowing that cotton plants mature quickly,many soybean growerss in O plan to cease growing soybeans and begin raising cotton instead,thereby taking advantage of the high price of cotton to increase their income siginifcantly,at least over the next several years.

Which of the following ,if true,most seriouslly weakens the plan's chances for success?

(B) tests of a newly developed,inexpensive pesticide have shown it to be the both environmentally safe and effective against the insects that have infested cotton crops.

(C)In the past several years,there has been no sharp increase in the demand for cotton and for goods made out of cotton.
答案选的是b ,想不明白怎么个削弱法

对，因为棉花mature quickly，所以一旦这种新药运用，马上就会翻身的，产量就回来了，价格和产量成反比啊。

而C说demand没有sharp上升，但是supply因为insect减少了，所以不weaken，相反，如果说demand有sharp的下降，说不定倒是weaken立论了