解析：in search of 寻找。

　　答案：in

　　2．You'd better watch ________ for any dangerous animals while crossing the forest.

　　解析：watch out for 当心，注意。

　　答案：out

　　3．All ________ once, the crowd burst into cheers.

　　解析：all at once 突然。

　　答案：at

　　4．The creature can grow to two metres ________ length.

　　解析：in length 长。

　　答案：in

　　5．After taking the medicine, he gradually picked ________.

　　解析：pick up 康复。

　　答案：up

　　6．From then ________， the Greens lived a happy and cozy life.

　　解析：from then on 从那时起。

　　答案：on

　　7．The people ________ board all lost their lives.

　　解析：on board 在船上，在飞机上。

　　答案：on

　　8．He broke ________ his left arm in the accident.

　　解析：break off 折断。

　　答案：off

　　9．My sister cares ________ playing football.

解析：care for 喜欢。