how后面能否加be动词

how后面能否加be动词
考试时的一篇完形填空,文章如下：
Mike and Eric are my best friends at school.We study in Class 3.The two boys are twins.But some student don't know that they are twins,because there are many differences between them.Mike doesn't look like Eric.Mike is short,but Eric is tall.Mike is thin ,but Eric is a little bit heavy.Mike doesn't wear glasses.His twin brother is a boy with glasses.Mike is a quiet boy.He doesn't like to talk with others,but Eric is always talking.Eric likes to tell jokes.And he is very popular in our school.We ofter say him,"Hi,Eric.40 is te joke today?"Isn't that iteresting.
中间的40是一个空往里面填选项,如下：
A.Who B.Why C.How D.What
我选了D对了,后来发卷子的时候一群人说C搞的我们的老师也为难,请问有没有How+be的用法