I have one last Riddler Trophy to collect, but I simply can't figure out how to get it. In The Bowery section, to the very northwest of the Museum (on the roof) is a Riddler Trophy in front of a cage. When I step on the switch here, the cage closes and I'm supposed to wipe out the three question marks inside the cage. My guess is that I'm supposed to use the explosive gel, but when I click on detonate they (a) don't detonate in the order I want them to and (b) they detonate before the second question mark is even activated. I've tried all the other gadgets but I can't figure it out. How in the world do I solve this one?