ask for help后面加什么介詞

　　Learn to ask for help when needed.

　　學會在需要的時候尋求幫助。

　　Only ask for help when you need it.

　　只在需要幫助的時候才去尋求幫助。

　　Whom did the baby snail ask for help, his mother or father?

　　小蝸牛向誰求助了，他的媽媽還是爸爸？