Emails have the following features which one is WRONG? ()

Emails have the following features,which one is WRONG？ ()

A、They are more informal.

B、Headings are put in automatically and replace addresses and signatures.

C、Often no salutations or closings are used.

D、Separate files and documents cannot be attached.

请帮忙给出正确答案和分析，谢谢！