根据材料内容，选择正确答案。

Dear Frank,    

    Thank you for your postcard. I'm very glad you are having a good trip now. Ottawa is very beautiful in May. I hope I will have a chance to go there one day.

    Our summer holidays will start next month. I enjoy my summer holidays, although(虽然)it's hot at that time in America. During summer holidays，I can swim in the sea and go fishing. It's so cool. And I am going to our hometown near Chicago with my sister Linda，and my parents agree(同意). Our uncle lives  there，so Linda and I can stay with him.

    We are not going to take a plane. We are going there by train. I am going to fish there. And Linda likes painting. She is going to do some painting there. I'm sure we'll have a good time.

                                                            Sincerely,

                                                              Jack

I am sure that he is calm enough ____ this problem.

一辆汽车正在笔直的公路上以72km/h的速度匀速行驶，司机看见红色交通信号便踩下制动器（司机的反应时间不计），以后汽车开始减速，设汽车做匀减速运动的加速度大小为5m/s²，则，开始制动后，