Chinese scientists have successfully cloned （克隆）two ．They are named Zhong Zhong and HuaHua． （monkey）
Children will be in danger in a swimming pool if their parents don't take good care of ．（they）
The second﹣hand car is much （cheap） than the new one．
This is Mary's （three） time to visit the beautiful city．
Teenagers should learn how to protect （they） from all kinds of dangers．