单选题
Over the last three years a scientist had an average (arithmetic mean) yearly in come of $45,000. The scientist earned 1 
times as much the second year as the first year and 
times as much the second year as the first year and
【正确答案】
D
【答案解析】[解析] 设这位科学家第一年的收入为x美元，根据题意可得：
所以他第二年的收入为
所以他第二年的收入为