请根据句意从方框中选择合适的单词，并用其适当形式填空，使句子通顺．
| easy serious I seventy speak |
51．-Can I use your bike this afternoon？
is under repair．
-No problem．Here's the key．
52．Cell-phones have made our lives much
，but they also bring us some troubles．
53．Mr．Black has made several
on how to fight against pollution．
54．Since your leg is
hurt，you have no choice but to lie in bed．
55．My family are going to have a party to celebrate my grandpa's
birthday．