On the morning of November 18, 1755, an earthquake shock Boston． John Winthrop, a professor at Harvard College, felt the quake and awoke. “I rose”, Winthrop wrote, “and lighted a candle, looked at my watch, and found it to be 15minutes after four.” John Winthrop hurried downstairs to the grandfather clock．It had stopped three minutes before, at 4:12．Except for stopping the clock, the quake had only thrown a key from the mantel（壁炉台）to the floor．
The clock had stopped because Winthrop had put some long glass tubes he was using for an experiment into the case for safekeeping． The quake had knocked the tubes over and blocked the pendulum（钟摆）．Winthrop, therefore, had the key on the floor． The quake had thrown it forward in the direction of the quake’s motion by a shock coming from the northwest, perhaps in Canada．
What does “it” in the second paragraph refer to?

A．the floor

B．the key

C．the clock

D．the case

The text suggests that     ．

A．earthquakes are common in Boston

B．John Winthrop was a scientist

C．John Winthrop often had difficulty in sleeping

D．Harvard College was a center for earthquake researching

Which of the following is true?          .

A．Some tubes were broken into pieces and stopped the old clock

B．The professor lit a candle because of power failure

C．Boston is in the southeast of Canada

D．The shocks were slight in one way

John Winthrop put some tubes into this clock case because        ．

A．he wanted to do an experiment

B．he thought an earthquake was probably to happen soon

C．he thought it safe for them to be put there

D．he wanted to record the exam time of the earthquake．