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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