-------,we didn't go swimming .A Because it rained.B Because of the rain

答案选 A Because it rained
后面的句子必须是一个完整的句子，但这里B单独构不成一个句子。

B 首先这是一句话，而且后半句没有连词，所以横线部分不能是一句话，所以只能选择B这个介词短语。
英语里面，在一个句子里面，不能同时出现两个没有连词连结的俩个句子。

A, B都对，A 是because引导的原因状语从句，B是Because of介词短语。

B

严格地说,B是正确的答案,因为：
1 AS / SINCE it rained,we didn't go swimming
2 We didn't go swimming BECAUSE it rained.
3 We didn't go swimming ,FOR it rained
=== We didn't go swimming BECAUSE OF the rain .
(请比较一下连接词的位置)

选择A，因为B单独构不成一句句子，如果要选B，需改成we didn't go swimming because of the rain.

答案选A，后面的句子必须是一个完整的句子，所以B一个短语显然不合适