B 吧.. 这个句子是强调句啊！！It is because English is very useful that we work hard at it.[强调句：It is/ was + 被强调部分（通常是主语、宾语或状语）+ that/ who（当强调主语且主语指人）+ 其它部分。]强调句还有个鲜明的特点，就是把结构 It is/ was 和taht/who去掉后，句子仍成立。看看这句子，把结构去掉后变成：We work hard at it because English is very useful。很显然句子成立！！...
