________ is synthesized from tryptophan

What is the major reason that a healthy couple having intercourse regularly is not likely to conceive a child immediately?

a) Sperm must make a journey filled with hazards. b) Sperm live for only a few seconds after ejaculation. c) An ovum actively resists fertilization by sperm cells. d) Most ova have not reached a level of maturation that allows for fertilization.

The retrograde amnesia associated with closed-head injury has been frequently studied in laboratory animals by

A) hitting them on the head with a little rubber hammer. B) administering acetylcholine agonists. C) administering electroconvulsive shock. D) using multiple-trial learning tests that must be learned over several days. E) both B and D