题目内容：
通读下面的短文，掌握其大意。从每小题的四个选项中选出可填入相应空白处的最佳选择，并把它前面的大写字母填入左边括号里。 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 professor， who _23_ in the university just up the road from Mr． Jones‘ house， coming the direction of the town． He had grey hair thick glasses， and was _24 an umbrella， a morning newspaper and a bag． Mr． Jones thought that he must have _25_ by the night train _26_ taking a taxi．
Mr． Jones had a big tree in his garden， and the children had tied a long _27_ to one of the branches， so that they could swing on it．
Mr． Jones saw the professor _28_ when he saw the rope and looked carefully up and down the roaD．When he saw that there was _29_ in sight， he stepped into the garden （there was no fence）， put his umbrella， newspaper， bag and hat nearly on the grass and _30_ the rope． He pulled it _31_ to see whether it was strong enough to take his weight， then ran as fast as he could and swung into the _32_ on the end of the rope， his grey hair blowing all around _33_． _34_ he swung， sometimes taking a few more _35_ steps on the grass when the rope began to swing too slowly for him．
At last the professor stopped， straightened his tie， combed his hair carefully， put on his hat， picked up his umbrella， newspaper and bag， and continued on his way to the university， looking as quiet and correct and respectable as one would expect a professor to be． （ ） A． because
B． as
C． so
D． for
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