SAT句子提升的一道题求教
Bypaintingthemthisafternoon,thewallwouldbecompletelydrybytomorrowevening.
改bypaintingthem这一句.我选的是Afterpaintingthem,答案是Weretheytobepainted.
为什么这么选?我选的为什么不对?也请帮我分析一下正确答案的结构.