Dark clouds____ a sign of rain or snow. A is B was C are D w

选C
前面clouds 是复数,所以谓语用复数形式,a sign of 表示 ...的一种象征

Dark clouds____ a sign of rain or snow. A is B was C are D were 选哪个？为什么？

楼上错了。
正确答案是: A is 。

因为 a sign 是单数，所以要用 is，(is a sign 是一个预兆) 了。

很希望会对你有帮助。