A block of mass m is suspended by a light thread from an elevator. The elevator is accelerating upward with uniform acceleration a. The work done by tension on the block during t seconds is (u = 0)
A. `(m)/(2)(g+a)at^(2)`
B. `(m)/(2)(g-a)at^(2)`
C. `(m)/(2)gat^(2)`
D. 0