At last, the teacher made the students _______ in the classroom, so that he can have the lesson. [ ]

A. stop to talk

B. to stop talking

C. stopping talking

D. stop talking

已知空气中氧气的体积分数为21％，小兰同学用下图所示的装置进行验证，实验后发现测得的氧气体积分数小于l／5，请你帮她分析可能造成这种结果的原因(答两点)：

①                        、 ②                     。