It's a fine day.Let's go fishing,shall we?

It's a fine day.Let's go fishing,shall we?
It's a fine day.Let's go fishing,shll we?
答案写的是shall we?
我写的是isn't we?麻烦谁解释下,明明前面有be词（is)不用,非要加个shall.请解释的通俗点,本人底子差.
23.The building _____ roof we can see is the Western Church.
A.its B.which C.whose D.who's
我选的B。明明问的是建筑又不是人，怎么会选C啊？
The first software of Windows _____ by Bill Gates became very popular with computer lovers immediately.
A.made B.had made C.was made D.had been made
我选的D。若答案D为has been made是否正确？