This river is seriously polluted now,but once inside it ______. [ ]

I guess you can buy such a smart phone for 300 dollars—I could just two months ago，but the price ________ by 20%

31Nobody believed his reason for being absent from the class ______ he had to meet his uncle at the airport

The journey was adventurous and they went through great hardship, but it .

Can you join the ________ club-Yes. I can swim very ___________.