大全285，难道“must”可以随便加？

285.Even though the state has spent ten years and seven million dollars planning a reservoir along the Ubi River, the project will have to be abandoned as a result of the river becoming so heavily polluted.
(A) will have to be abandoned as a result of the river becoming so heavily polluted
(B) is to be abandoned on account of the heavy pollution which the river received
(C) had to be abandoned because the river had received such heavy pollution
(D) has to be abandoned because of the river and its heavy pollution
(E) must be abandoned because the river has become so heavily polluted

大全解释：

285, sec18-18

A) as a result of the river becoming so heavily polluted不能准确表达原意。as a result of后一般接实义名词如pollution, 这里成了as a result of the river, becoming so heavily polluted是修饰river的; 如把the river becoming看成the river’s becoming那也不如用句子表达意思清楚。

B) is to be abandoned和on account of the heavy pollution which the river received都不能正确表达原意。on account of表原因一般不如because从句。
C) had to和had received时态与前has spent不匹配。
D) because of the river and its heavy pollution不能清楚地表达原意。
E) best.

疑问：

1 难道 “MUST”可以随便加？ 不改变句意吗？ 我记得情态动词不可以随便加或改，目前为止只看到：may 和might互换。

2 我本题用什么时态最合适？

谢谢各位