**Q. What constitutes a "strain"? I gather it is more than just color. Does it also include the class? Species? Both?
A. Strain is a sub-division of species. Dogs are a species, while Dobermans and Poodles are strains. Species is a biological reality, while strain may be a matter of opinion, or marketing.

**Q. I am not clear on the explanation of the number of electrons associated with the radical molecule OH(with a dot) in quiz question 7.
A. Oxygen had 6 and hydrogen has 1, so OH must have 7. Now the hydroxyl ion, OH^- has an extra electron, that makes it a charged particle; it has 8 electrons. It often steals that electron from a different hydrogen, leaving that without any electrons, just a poor proton, H⁺

Q. In Sub module 1B the point is made that any substance in the right amount can be considered a poison. The one chart showed Oxygen as being a potential cause of birth defects. I recognized the other four examples, even water causing cell breakage, but don't remember hearing about Oxygen. Under what circumstances would Oxygen cause a birth defect?
A. High pressure oxygen is directly toxic, that's why it isn't used for divers. There is some evidence that maternal hyperoxia can cause birth defects in the fetus.

Q. Question #8 in the quiz addresses the definition of "Highly Toxic" and the type of rat used in the testing. Re-reading the definition, I agree it only says laboratory white rat. In Stine, however, in discussing the LD50, they refer to the white Norway rat as being the standard laboratory animal used. In the question you refer to "strain" of rat used. I gather that there are different strains of Norway rat? After re-reading the biology portions, I'm not sure I fully understand breeding of strains of animals.
A. Turns out that "Norway Rat" is a species, Rattus norvegicus. Most lab rats are technically some strain of the Norway Rat. Note many have been bred for many generations for some special characteristics and might appear very different than the archetype Norway Rat, and from other strains.