白勇书上条件状语从句16题

ljwqlcl881703

If the new airboat does what it is to be doing-----travel
at high speeds undeterred by sandbars crocodile-infested mudflats, or
marshy hippo haunts---it could revolutionize transport on the
2,900-mile-long Congo River.
A. If the new airboat does what it is to be doing
B. If the new airboat does what it is supposed to do
C. If it does as the new airboat is supposed to do
D. Doing what it is the new airboat is supposed to do
E. Doing what the new airboat is to be doing
选B  后面主句用的为啥是could  could这里是个情态动词么？ 为啥不用主将从现的形式？ 希望能帮忙分析下句子！谢谢！

ljwqlcl881703

帮忙也翻译下if 那句话的意思 thx