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When it______ air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.

A. refers to                B. comes to               C. happens to           D. speaks to

When it______ air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.

A. refers to            B. comes to           C. happens to         D. speaks to

When it______ air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.

A. refers to            B. comes to           C. happens to         D. speaks to

When it______air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.

       A. refers to        B. comes to         C. happens to        D. speaks to

When it______ air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.

A. refers to            B. comes to           C. happens to         D. speaks to

When it    air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution．

A．refers to        B．comes to          C．happens to         D．speaks to

When it    air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.

    A. refers to        B. comes to         C. happens to        D. speaks to

When it    air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.

    A. refers to        B. comes to         C. happens to        D. speaks to

When it______ air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.

A. refers to　　　 B. comes to　　　 C. happens to　　 D. speaks to

When it______air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.