As more and more cars are produced and used, so the _____ from their exhaust-pipes contains an even larger volume of poisonous gas.

Animals should not be locked in cages; they should be in their normal _____.

I’ve listened to Professor Smith’s talk about medicine,but I still haven’t got a good understanding of its key _________.

There might be life on other planets, but so far scientists haven't found enough ________ to prove it.

After ___ absence, she found her hometown greatly changed.