16．It's ten years _________ the scientist set out to discover the valuable chemical．（　　）
A．whenB．thatC．beforeD．since

分析 自從這個科學(xué)家著手開始尋找有價值的化學(xué)物質(zhì)已經(jīng)有10年了．

解答 答案：D．根據(jù)選項since自從…，before在…前，when當(dāng)…時，再結(jié)合句意，自從這個科學(xué)家著手開始尋找有價值的化學(xué)物質(zhì)已經(jīng)有10年了，可知此處缺少了一個引導(dǎo)時間狀語從句的連接詞，故D 正確．句型It is/has been+一段時間+sincee…可翻譯為，自從…已經(jīng)多長時間…

點評 本題考查時間狀語從句的連接詞，在充分分析句型再結(jié)合語義的基礎(chǔ)上便能選出正確答案，同時要注意其他選項的意思

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