一道完形填空题If we take a close look at successful language leaner

一道完形填空题
If we take a close look at successful language leaners,we may discover a few techniques(技巧) which make language leaning easier for them
16.____,successful language learners .They ao not depend on books or teachers;they 17.____ their own way to learn the language.They try to find the patterns and the rules for themselves 18.____ waiting for the teacher to explain everything .They are good guessers who look for clues and form their 19.____ conclusions which are very different from others'
16.A As usual B First of all C By the way D So far
17.A explain B make C discover D change
18.A instead B after C because of D from
19.A correct B own C only D main
16题答案给的B 但是我觉得A As usuall 通常也可以啊?
为什么