read the follwing passage, then choose the best answer (A, B, C or D) to fill in the blank
It can be shown in facts and figures that cycling is the cheapest, most convenient, and most environmentally desirable form of transport (17)_________ towns, but such cold calculations do not mean much on a frosty winter morning. The real appeal of cycling is that it is so (18) _______. It has none of the difficulties and tensions of other ways of traveling so you are more cheerful after a ride even through the rush hour. The first thing a non-cyclist says to you is: "But isn't it (19) ______ dangerous?" it would be foolish to deny the danger of sharing the road with motor vehicles and it must be admitted that there are an alarming (20) ......of accidents involving cyclists. However, although police records indicate that the car driver is often to blame, the answer lies with the cyclist. It is possible to ride in such a way as to  reduce risk ta a minimum.
17.A. in              B. at             C. to                 D. on
18.A. boring       B. careful     C. enjoyable     D. excited
19.A. expectedly  B. strangely    C. terribly    D. comfortably
20.A. size           B. number       C. deal         D. digit