設隨機變量X的分布律為P{X=k}=C＊(3^k)/k!，k=0,1,2,..., 若E(X)=9/2，則D(X)= _______．

題目：設隨機變量X的分布律為P{X=k}=C*(3^k)/k!，k=0,1,2,..., 若E(X)=9/2，則D(X)= _______．
A. 9/4
B. 9/2
C. 27/4
D. 81/4