However careful one may be, he cannot possibly listen carefully to everything that he hears. There are 11 of reasons for this. ①One of them is the overload（超负荷）of messages most of us 12 each day. In addition to the numerous hours we 13 hearing other people speak, we may spend several hours listening to the radio or sitting in front of a television set.② 14, it is impossible to focus our attention completely on what is said; our mind might be 15 elsewhere. ③Preoccupation（先占，当务之急）with our personal concerns is 16 reason we don’t always listen carefully. A romance 17 sour or a good grade on a test may take prominence in our mind even as 18 is speaking to us. ④Furthermore, we are surrounded by all kinds of noises which interfere 19 listening. For example, voices at a party or 20 of traffic may simply make it difficult for us to catch everything that is being said.
11. A. the number
B. a number
C. number
D. numbers