有一句谚语“捡了芝麻，丢了西瓜”，意思是说有些人办事只抓住一些无关紧要的小事，却忽略了具有重大意义的大事．据测算，5万粒芝麻才约200克，那么，一粒芝麻约为（    ）克．（请用科学记数法来表示）

Smith ______ a book about China last year, but I don't know whether he has finished it.

A．has wrote

B．wrote

C．had written

D．was writing