题目

  When I was at university，I studied very hard．But a lot of my friends did very little work．Some did just enough to pass exams．Others didn’t do quite enough．Fred Baines was one of them．He spent more time drinking in the Students’ Union than working in the library．    Once at the end of the term，we had to take an important test in chemistry．The test had a hundred questions. In my room the night before the test，Fred was watching TV．Fred usually worried a lot the night before a test．But on that night he looked perfectly calm．Then he told me of his plan．“等比数列公比不为1，其前n项和、前2n项和、前3n项和分别为P、Q、R，则（ ）A．P+R=2QB．Q2=PRC．R=3（Q-P）D．P2+Q2=P（Q+R）