【正确答案】 A

【答案解析】由已知得 =α[D(X1)+E2(X1)]+(1-α)[D(X2)+E2(X2)] =α+5(1-α)=5-4α， D(X)=E(X2)-[E(X)]2=5-4α-(1-α)2 =4-2α-α2=5-(α+1)2．