课外探究小组的同学们为探究一定量的高锰酸钾所制得氧气的体积和细铁丝在氧气中燃烧的实验现象,进行了如图实验,请你参与他们的实验并回答有关问题:
I'm very busy. I have mountains of work to do.
——How long have you ________ your penfriend?——For five days. I received it last Sunday.