[jay_shark]

There is a 1/10 chance that you have both parts belonging to doors A and B .

P(AC+AD+BC+BD+CD) =9/10
we know P(AC) =P(AD)=P(BC)=P(BD) but we have to check P(CD) .
Also P(AC) = 2/5*3/4*1/2 = 0.15

So we have 0.15*4+P(CD)=9/10
P(CD) = 0.3 .

You should select Doors C and D .