設(shè)隨機(jī)變量X的分布律為P{X=k}=a＊(3^k)/k!，k=0,1,2,..., 且E(X)=3，則D(X)= _______．

題目：設(shè)隨機(jī)變量X的分布律為P{X=k}=a*(3^k)/k!，k=0,1,2,..., 且E(X)=3，則D(X)= _______．
A. 3
B. 6
C. 9
D. 18