With his attention________on the shops along the roadside，he knocked an old man down while riding to school.

A  concentrating     B．concentrated     C．to concentrate      D．concentrates

答案　B

解析　考查词组，故选B项。

 Both _______ and _______ laughter do harm to your health.

A. burst into tears; burst into

B. to burst into tear; to burst into

C. bursting into tears; bursting into

D. tear; /

 I ______ laughing when my daughter made a face.

Seeing the stranger _______, the frightened baby couldn’t help ______ crying.

A．burst in; bursting into                    B．burst into; burst out

C．break in; bursting out                    D．burst into; burst out

 I_______ laughing when my son made a face.