有个英语中的小问题

Question

这是一道改错题。原题是Class is begin。why not stand up？

Class is beginning。why not stand up？这样改对吗？
有的人说Class is begin。why not standing up？才对
究竟是谁写的对，还是都错了？

Answer 1

why not do sth = why don't you do sth
begin 是动词 ，跟在is 后肯定要改变形式
class begins 或者 calss is beginning 都可以
所以：
Class is beginning。why not stand up？
Class begins。why not stand up？

Answer 2

Class is beginning。why not stand up？这样改对

Answer 3

习惯性都会说Class begins. Why not standing up?