单选题 Given P(X=2) =0.3, P(X=3) =0.4, P(X=4) =0.3. What is the variance of X?
  • A.0.3.
  • B.0.6.
  • C. 3.0.
【正确答案】 B
【答案解析】The variance is the sum of the squared deviations from the expected value weighted by the probability of each outcome.
The expected value is E(X) =0.3×2+0.4×3+0.3×4=3.
The variance is 0.3×(2-3)2+0.4×(3-3)2 +0.3×(4-3)2=0.6.