Some people do not like anything to be out of place．They are never late for work ；they return their books to the library on time；they remember people' s birthdays and they pay their bills as soon as they arrive．Mr．Brown is such a person．Mr．Brown works in a bank, and lives on his own．His sister lives in the next town with her husband, and her son, Mark．Mr．Brown doesn' t see his sister or her family from one year to the next, but he sends them Christmas cards, and he hasn' t forgotten one of Mark ' s seventeen birthdays．Last week Mr．Brown had quite a surprise．He drove home from the bank at the usual time ；driving neither too slowly nor too fast, he parked his car where he always parked it, out of the way of other cars, and he went inside to make his evening meal．Just then there was a knock at the door．
Mr．Brown opened the door to find a policeman standing on the doorstep．
"What have I done wrong?" Mr．Brown asked himself．"Have I driven on the wrong side of the road? Has there been some trouble at the bank? Have I forgotten to pay an important bill?"
"Hello, uncle," said the policeman, "My name' s Mark．"
Mr．Brown sees his sister_______．
A.very seldom
B.only at the end of the year
C.only at Christmas
D.on Mark' s birthday every year