单选题
| k | ak | pk |
| 1 | 100 | 0.10 |
| 2 | 200 | 0.25 |
| 3 | 300 | 0.20 |
| 4 | 400 | 0.25 |
| 5 | 500 | 0.20 |
If in an experiment the probabilities of obtaining the values a
1，a
2，a
3，a
4， and a
5are p
1，P
2，P
3，P
4 and P
5，respectively，then the expected value is defined as a
1p
1+a
2p
2+a
3p
3+a
4p
4+a
5p
5．For the values and their corresponding probabilities in the table above，what is the expected value?
A. 350 B. 320 C. 300
D. 270 E. 250