读“某区域地质剖面简图”，回答问题。

根据句意，用括号内所给动词的适当形式填空。(必要时加助动词或情态动词)
____ Mr Zhang ______(teach)us English three years ago.
____ Peter turned off the computer after he finished______ (write)an e-mail.
____ —Where's Miss Wang?一Look，she______(talk)with the students.
____ Mr Liu isn't here now．He______ (go)to Beijing.
____ The children will go to the zoo if it______ (not rain)this Saturday.
____ There______ (be) a meeting in our school tomorrow afternoon.
____ Mr Green______(watch)TV at this time last night.
____ You’d better______(eat)less food before you go to bed.
____ My teacher told me that light______(travel)much faster than sound.
____ English______ (speak) by many people in the world.

石英砂是制造玻璃的重要原料，其主要成分是二氧化硅（SiO₂)，其中硅元素的化合价为（）

早春播种以后，农民常用“地膜覆盖”，以促进早出苗，其原因是（  ）