The following line graph gives the ratio of the amount of entry to the amount of exit of a specific molecule from a cell, over the time period 1-7 hours. If the entry at 4 hours was 250 units and the total exit at 4 hours and 5 hours together was 500 units, then the entry at 5 hours was:
a. 250 units
b. 300 units
c. 357 units
d. 420 units