An account accumulates accoraing to the accumulation function, a(t) t2 +1 50 for the first four years and at a force of interest of 0.01t thereafter. A deposit of 700 is made into the account at time t. At time 10, the account has grown to 1,200. Calculate t. D 2.9 B 3.3 3.7 D 4.1