A．从下列单词中选择恰当的词填空，每个词只能用一次。

however   if            beneath      intend        for as           meanwhile     proving      enabled        therefore

Six men have been trapped in a mine （51）seventeen hours. If they are not brought to the surface soon they may lose their lives.  （52） , rescue operations are  （53） difficult.   （54） explosives (炸药) are used, vibrations (振动) will cause the roof of the mine to collapse. Rescue workers are  （55） drilling a hole on the north side of the mine. They  （56）  to bring the men up in a special capsule. If there had not been a hard layer of rock  （57） the soil, they would have completed the job in a few hours.  （58）  it is, they have drilled for sixteen hours and they still have a long way to go.  （59）  , a microphone, which was lowered into the mine two hours ago, has  （60） the men to keep in touch with their closest relatives. Though they are running out of food and drink, the men are cheerful and confident that they will get out soon.They have been told that rescue operation are progressing smoothly. If they knew how difficult it was to drill through the hard rock, they would lose heart.

（51）