本题考查双重关系分句。双重关系分句指一个先行项之后跟有两个关系分句的语法现象。所给的两个句子的主语都是The monsoon season，所以可以把其中一个句子用关系词which引导放在The monsoon season后面作定语从句。两个分句不分先后，哪一个作主句或者作从句都可以。所以答案可以是The monsoon season which causes heavy rains occurs in Bangkok every year.或者The monsoon season which occurs in Bangkok every year causes heavy rains. 

句意：每年曼谷都会出现导致暴雨的季风季节。