2. 外语
  3. After inv estimating five residential gas explosions that killed four people in the Kansas City--Topeka area in 1988 and 1989, t

After inv estimating five residential gas explosions that killed four people in the Kansas City--Topeka area in 1988 and 1989, t

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问题      After inv estimating five residential gas explosions that killed four people in the Kansas City--Topeka area in 1988 and 1989, the National Transport action Safety Board has【C1】______ that automatic shut -off valves be required when home gas lines are installed or 【C2】______ The valves stop 【C3】 ______ and potentially【C4】______ surges in gas flow, such as would occur 【C5】______  a contractor hit and ruptured a pipe.
     The NTSB labeled the report" urgent",【C6】______ it doesn’t rate the risk
of surges high enough to ten consumers to【C7】______ $ t000 or so to dig up old lines and【C8】______ the $15 to $20 valve. A bigger【C9】______ to homeowners, says the NTSB, is the danger of slow 【C10】______ from pipe cur rosin,【C11】______ the valve would not prevent. Symptoms of a possible gas【C12】______ under your property include a sulfur smell and brown spots or dead【C13】______  in your yard. Call your gas company【C14】______ two of the Kansas City-area explosions【C15】______  had gas odors been【C16】______ And before【C17】______ start construction in your 【C18】______ , ask your gas company to【C19】______ mark the position of pipes【C20】______  your house.
【C5】
选项 A、if
B、when
C、as
D、since
答案A
解析 根据语法，此句为虚拟语气，故应填A if。
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