选were
表示与现在事实相反的情况：
从句：If 主语+过去时 主句：主语+should/would/could/might+do
eg1．If I were you,I would take an umbrella.如果我是你,我会带把伞（事实：我不可能是你） 　　2．If I knew his telephone number,I would tell you.如果我知道他的电话号码,我就会你.（事实：不知道）
这道题是与事实相反的,因为他不可能是超人,所以填Were(Be动词的过去式在虚拟语气里一般都用were)