He began to learn English _____ (in / at) the age of six
句意：他六岁开始学英语。at the age of…在几岁，固定搭配，故选 at。
2018年12月，张阿姨把4000元的存入银行，定期三年，年利率是2.75%到期后，应得利息______元.
He began to learn English _____ (in / at) the age of six
Her mother will come back _____ (after / in) five days.
Autumn comes _____ (before / after) summer.
What do you often do _____ (at / in) weekends?
We will go to the cinema _____ (in / on) Sunday evening.
Do you know _____ (why / what) you are studying for?
I don’t know _____ (how / why) so many people are looking at him.
They don’t know _____ (if / whether) to go or wait.
I really don’t know _____ (how much / how soon) the bridge will be finished.
