a company's sales executive gets monthly salary as two parts.  one is fixed and other depends upon the sales he achieved(i.e commission  on sales). on the first three months he got 30% , 20% , 10% of the sales achieved and the fixed amount was 1000 , 2000 , 3000 respectively. then what can be said about his sales achieved

a)it was increasing continously for 3 months

b)it was decreasing continously for 3 months

c)it cannot have gone up in 2^_nd month and then gone down in 3^_rd month

d)it increased in 2_^nd month and then gone down in 3_^rd month