[单选题]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 roa D．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