a set of keys后加is还是are

　　例句：

　　As it happens, I have a spare set of keys in my office. 碰巧我在办公室有一套备用钥匙。

　　My mom gave me a set of keys. 我妈妈给了我一串钥匙。

　　A set of keys is on the desk. 有串钥匙在桌子上。

　　Stranger: Excuse me, did you see a set of keys? 陌生人：你好，你看到一串钥匙了吗?