题目内容：
Martin Henfield talks about some of his experience(经历) as a twin: when we were smallmy mother dressed us 21 the same clothes. That was bad enough and we didn't like it. Butwe went on our first camping trip, it was even 22 We were only ten years old, and while 23 went into their sleeping bags for the 24 , we were not happy to snuggle(偎依) inside a double sleeping bag my mother made for us. At school our classmates 25 us Henfield One and Henfield Two, so people 26 even see our difference according to our initials because 27 of us were M. O. It was onlywhen I went to 28 and began to have my own friends that I started to feel my own freedomof identity( 身份).
Before I went to college, during my secondary school 29 , I 30 to a job on a building site. My twin brother, Mike Henfield, didn't work. One day I asked my boss, "Can I have aweek 31 ? Certainly," he said, "but you won't have the job when you 32 back. " Ididn't want to 33 the job. So on Monday morning, Mike went there in my 34 , jacketand hat and he worked for me for one week. 35 of them knew the difference.
根据以上内容,回答题.
第21题应填入( ) A．in
B．for
C．on
D．with
参考答案：
答案解析：