Bob, can you see the boys over there are my cousins. Let’s go and say hello to them.

Listen! Do you hear someone forhelp

下列各句中，表达得体的一句是（ ）

Two years ago, three scientists traveled to the Antarctica to explore a secret world hidden _________ the ice

”I’m sorry I forgot______ the letter for you!”“It doesn’t matter, I’ll post it myself.”