-How long will it be ______ Chinese astronauts set foot on t

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-How long will it be ______ Chinese astronauts set foot on the moon?-Maybe within ten years.A .since B .before C .until D .when

since是从过去的某一时间算起 until直到 when当什么时候很显然其他词语说不通
再问： before在这里是什么用法？
再答：整句话的意思是还有多久中国宇航员才能踏上月球 until用于肯定句中，主句的谓语动词必须是延续性动词，表示这一动作或状态一直延续到until所表示的时间为止，显然该句的动词是be，不由延续性

-How long will it be ______ Chinese astronauts set foot on t I don't know how long it will be ______ I can return to my h HOW LONG WILL IT BE? Nobody knows exactly how many years it will be--Chinese foot How long does it take to get to school on foot?Marry wants t How long do you think it will be ______ the economy revives? It______some Chinese astronauts will fly into the space in t how long will be How long was it be,since i held you near?How long will it be who knows how long it will be lasted 为什么是How long do you think it will be ... How long does it take you to get to school on foot and _____