Task A has the following details：Optimistic estimate=30, pessimistic estimate=40, most likely estimate=35. Task B has the following details：Optimistic estimate=32, pessimistic estimate=37, most likely estimate=34. Task C has the following details：Optimistic estimate=25, most likely estimate=35, pessimistic estimate=45. Which task has the highest standard deviation （）
A. Task A.
B. Task B.
C. Task C.
D. Task A & Task C.