what’s （A）his(B) phone number?It's(C) 6374218 A、B、C 中 哪个是错误的

What's his phone number.It's 6374218.应是句子开头首字母大写,如果是WAS.C也要改.
再问： 可不可以把It’s 改成 his phone number 句子就是 his phone number is ……
再答： his phone number is ......没错。但It's 中的IT 就代表了HIS PHONE NUMBER. 没必要改