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

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)