The traditional approach __________ with complex problems is to break them down into smaller，more easily managed problems.

_________, when Marx was already ________, he began to learn Russian.

Listen! Is Mary singing in the next room—No. It _______ be Mary．She has gone to Shanghai for an important meeting.

The fog and haze(雾霾) in Beijing is getting worse and worse.— How terrible! We should try our best to protect the ___________.

Children like ________ Monkey King.