The hormone that was injected into the blood of a mouse is produced by the placenta during pregnancy. With this idea, we can cross out choices 2 and 4 since they are not related to the question. Now, we have a positive and a negative answer (choices 1 and 3). If the hormone is produced during pregnancy, this means that the hormone is a way for us to see if the "baby" is fertile. Antibodies are produced to stop the hormone from affecting the mouse's system, so the antibodies are preventing fertilization. Choice 3 is the right answer.