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!