we must pick up all the litter --------- the campsite after

we must pick up all the litter on the campsite after the barbecue。我们得把烧烤後留下的垃圾从营地上全部收拾走。（at用在说营地处有什麼叙述者基本不在营地内,人已经在营地裏活动,指所在地周围存在什麼用on）you should wash silk clothes with hand丝质衣服要用手洗。everything in the school library is put in good order学校图书馆所有东西被整理的井井有条。we are busy with a quiz for the festival我们为做一个节日的考查绞尽脑汁。busy with意思是“忙於”