A prezygotic isolating mechanism is a mechanism that causes something to happen, not allowing the organisms to breed before fertilization. I'll explain each one for you.
A - That is postzygotic because the organisms were not stopped before fertilization. The result is a sterile organism. B - Again, that is postzygotic because the organism was produced. C - This is prezygotic! The sperm dying shows that something is in action to prevent fertilization, meaning that it cannot happen. D - Fertilization was successful, so it is postzygotic.
ANSWER: C - the sperm dies when it reaches the female tract
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