Given a scientific question, generate a correct answer to it.

Example Input: In the past, what colliding with planet earth has caused many organisms to die off?
Example Output: asteroids

Example Input: What type of plants have vascular tissue but do not have seeds?
Example Output: seedless vascular plants

Example Input: Urea, ammonia and uric acid are types of what waste, excreted in different forms by different species?
Example Output:
nitrogenous waste