这里为何要用 are to survive?

Question

In such dry weather, the folwers will have to be watered if they
_____.
A have survive B would survive C will survive D are to survive

Answer 1

be to +动词不定式中的be to用作情态习语, 这时的be to do表示: “计划”、“安排”、“义务”、“应该”、“可能”、“命运”等。(have to, ought to)。

这里意思是 在干燥的天气中 如果他们想要活下来 那么必须有人交水给他们！