A tank contains 50 liters of oil at time $t = 4$ hours. Oil is being pumped into the tank at a rate $R ( t )$, where $R ( t )$ is measured in liters per hour, and $t$ is measured in hours. Selected values of $R ( t )$ are given in the table above. Using a right Riemann sum with three subintervals and data from the table, what is the approximation of the number of liters of oil that are in the tank at time $t = 15$ hours?
| $t$ (hours) | 4 | 7 | 12 | 15 |
| $R ( t )$ (liters/hour) | 6.5 | 6.2 | 5.9 | 5.6 |
(A) 64.9
(B) 68.2
(C) 114.9
(D) 116.6
(E) 118.2