题目内容：
通读下面的短文，掌握其大意。然后，从每小题的四个选择项中选出最佳选项。 Mr. Jones woke early one morning,before the sun had risen. It was a beautiful morning, 21 he went to the window and looked out. He was 22 to see a neatly-dressed and mid-aged profes sor,who 23 in the university just up the road from Mr. Jones' house,coming on the direction c the town. He had grey hair and thick glasses, and was 24 an umbrella, a morning newspaper an a bag. Mr. Jones thought that he must have arrived by the night train 25 taking a taxi.
Mr. Jones had a big tree in his garden, and the children had tied a long 26 to one of the branches, so that they could swing on it.
Mr. Jones saw the professor 27 when he saw the rope and looked carefully up and down th road. When he saw that there was 28 in sight, he stepped into the garden (there was no fence）put his umbrella, newspaper, bag and hat neatly on the grass and 29 the rope. He pulled 30 to see whether it was strong enough to take his weight, then ran as fast as he could an swung into the 31 on the end of the rope, his grey hair blowing all around his face. Backward and forwards he swung, sometimes taking a few more 32 steps on the grass when the rope bega to swing 33 slowly for him.
34 the professor stopped, straightened his tie, combed his hair carefully, put on his hat picked up his umbrella, newspaper and bag, and continued 35 his way to the university, looking as quiet and correct and respectable as one would expect a professor to be.
根据以上内容,回答题.
第21答案是（ ） A．because
B．as
C．so
D．for
参考答案：
答案解析：