+ By wind and rain;
+ Something that is found on a cloud;
+ fertilization occurs;
+ Males and females;
+ Something that comes from a volcano;
+ Something that is invisible;
+ Something that causes allergies;
+ sperm and egg fuse;
Q: You are presented with the quiz "What is required for plant reproduction?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "plant reproduction often requires pollen" and "hay fever or pollen allergy is the most prevalent allergy". So, what's the best answer to the question?
A:
Something that causes allergies