There are 24 solar terms in a year. Grain Rain often starts in Apirl,the           month of the year.

Third

fourth

fifth

sixth

考查数词。句意：一年有24个节气。谷雨通常在一年中的四月开始。空格前有定冠词the,与可数名词单数month，要用序数词。根据

Grain Rain often starts in Apirl，可知四月份是一年之中的第四个月，要用序数词fourth。故选B项。

There's no simple solution          this problem.

of

for

to 

about

考查介词。句意：这个问题没有什么简单的解决方法。此处是短语“

the solution to....的解决方法”，应用介词to。故选C项。

When Huawei P20 came out,         people couldn't wait to buy one in our town.

thousand

thousands

thousand of

thousands of

考查数词的用法。句意：当华为P20上市时，成千上万的人迫不及待的想在我们镇上买一台。thousnad意思是“千”，其前有具体数词修饰时，用其单数形式；当其前没有具体数词修饰时，用其复数形式，且与of 连用，thousands of“成千上万的”，这里没有具体数词修饰。故选D项。

Only when he returned ________ that the war had ended and the army had come back.

we tell

we told

did we tell

were we told

考查倒装句型。Only放在句首时，句子要用倒装语序；根据句意“我们被告知”可知，此处要用were we told。故答案选D。