If the First,second,third and Forth Worker Employed by the Firm

If the first,second,third and forth worker employed by the firm add 15,21,12 and 8 units of total product respectively,we can conclude that:

A) The marginal product of all four workers is 14.
B) The total product of two workers is 42.
C) That after the second worker marginal product declines.
D) That adding a forth worker will cause total product to decline.