A person invests money in three different schemes for 6 years, 10 years and 12 years at 10 percent, 12 percent and 15 percent simple interest respectively. At the completion of each scheme, he gets the same interest. The ratio of his investments is (a) 2:3:4     (b) 4:3:2     (c) 3:4:6     (d) 6:3:2