ANSWER:

Declarer is likely 5-5 in the majors, while dummy will have two or three hearts and short spades. The textbook lead is a low trump. I would go with that, but my second choice would be the ace and another trump to kill as many ruffs in dummy as I can. The low trump works if partner has the king, and is certainly more flexible, but the trump ace may be more effective if partner does not have a trump honor.