TASK DEFINITION: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
PROBLEM: A electron microscope is used to see extremely small objects.

SOLUTION: What type of microscope is used to see extremely small objects?

PROBLEM: Collision frequency is greater for homogeneous category of catalysts, which also tend to be more sensitive to temperature and more 'expensive'.

SOLUTION: Collision frequency is greater for what category of catalysts, which also tend to be more sensitive to temperature and more 'expensive'?

PROBLEM: Reversible reactions are indicated by opposite-headed kinds of arrows.

SOLUTION:
Reversible reactions are indicated by what kinds of arrows?