题目内容：

[单选题]D．

F．ind Which Direction Is South

D．o you have a good sense of direction? If not, please take with you a compass. But if you forget to take a compass, you can still find your way.

It’s never a good idea to imagine that the family member who was entrusted（委托）with the job of map-reading actually knows where the family is. You can tell by the slightly confused load on their faces that nothing on the ground seems to match the map. Never mind. The sun is shining and it’s still morning. If you don’t know the exact time, you can still find out where south is, but you’ll need to be patient.

①Find a straight sick and put it in the ground in a place where you can mark its shadow.

②Try to position the stick as vertically（垂直）as you can. You can check this by making a simple plumb line （铅锤线）with a piece of string and weight. You haven’t got any string? OK, use a thread from your clothes with a button tied at the end to act as a weight.

③Mark the end of the shadow cast by the stick.

④Wait approximately half an hour and mark the end of the shadow again.

⑤Keep doing this until you have made several marks.

⑥The mark nearest the stick will represent the shortest shadow, which is cast at midday, when the sun is highest in the sky and pointing to the exact south.

⑦Pick a point in the distance along the line between the shortest shadow and the stick.

⑧That point is south of where you are.

⑨Now you can turn the map, like you did before, and find which way you should be travelling.

68. To find the direction, we ought to be patient probably because

A． it is not easy to find a proper stick B. it is not easy to position the stick

C． it takes hours to make the marks D. it takes about half an hour to make the marks