[求助]大全test6-20，谢谢！

20.   Seven countries signed a treaty binding each of them to perform specified actions on a certain fixed date, with the actions of each conditional on simultaneous action taken by the other countries. Each country was also to notify the six other countries when it had completed its action.

The simultaneous-action provision of the treaty leaves open the possibility that

(A) the compliance date was subject to postponement, according to the terms of the treaty

(B) one of the countries might not be required to make any changes or take any steps in order to comply with the treaty, whereas all the other countries are so required

(C) each country might have a well-founded excuse, based on the provision, for its own lack of compliance依从, 顺从

(D) the treaty specified that the signal for one of the countries to initiate action was notification by the other countries that they had completed action（C）

(E) there was ambiguity with respect to the date after which all actions contemplated in the treaty are to be complete

c选项到底是什么意思呢？如何推出这个答案的阿？请高人指点下！谢谢啦！