B
David was earning money for college by working at a store in the late afternoon and early evening. On Wednesday night his boss asked him to work long past midnight, because another employee(雇员) got sick. Since David needed money for his car payment, he agreed. He did not tell his boss that he had an important test early the next morning.
On Thursday morning David slept through the alarm (闹钟). But, luckily, he was woken up by the telephone half an hour later. He arrived at the exam as the test papers were being handed out. However,he had missed the professor's instruction on how to do the test. David could not immediately begin to work on the test because he had to calm himself first. He was surprised by some of the material on the test. Ashe began to read and answer questions, he worried that he would not do well.
In fact, at the very beginning David made a poor decision. He didn't tell his boss that he needed a good sleep on the night before his big test. Being prepared rather than feeling tired and sleepy can make the difference in answering correctly one item (题目) or more. That correct item, for example, could make the difference in a failing score of 58 or a passing score of 60. So, don't take chances when the risks (风险) are high. Getting well prepared can make a difference. Set yourself up for success, and get enough sleep the night before a test.

How was David woken up on Thursday morning?