Step 1: Relation
$dU=nC_{v}dT$ for an ideal gas.
Step 2: Sign
Since $dU=dQ\lt0$, $dT\lt0$.
Step 3: Check
No work means constant volume, so V does not increase and T cannot rise.
Final Answer:
The gas cools, so T decreases.
\[ \boxed{\text{(D) }\text{T will decrease}} \]