QUES: reduce or eliminate the effects of harmful chemicals

correct the punctuation.

CORRECTED: reduce or eliminate the effects of harmful chemicals.

QUES: Powerful fence that keeps vegetation down

correct the punctuation.

CORRECTED: Powerful fence that keeps vegetation down.

QUES: In 1939 Liu joined the survivors of the Long March in Yenan

correct the punctuation.

CORRECTED:
In 1939 Liu joined the survivors of the Long March in Yenan.