Which of the following statements about mercury is false?
a. methyl mercury compounds are unusual in that they cannot cross the body's blood-brain barrier.
b. significant amounts of mercury are released into the environment during the smelting of other metals.
c. human exposure to mercury is primarily by eating marine animals containing high levels of mercury.
d. methyl mercury compounds are more toxic forms of mercury that readily enter the food web.
e. methyl mercury compounds persist in the environment for a long time.

Keeping in mind the life cycle of bacteriophages, consider the following problem: during the reproductive cycle of a temperate bacteriophage, the viral dna inserts into the bacterial chromosome where the resultant prophage behaves much like a trojan horse. it can remain quiescent, or it can become lytic and initiate a burst of progeny viruses. several operons maintain the prophage state by interacting with a repressor that keeps the lytic cycle in check. insults (ultraviolet light, for example) to the bacterial cell lead to a partial breakdown of the repressor, which in turn causes the production of enzymes involved in the lytic cycle. as stated in this simple form, would you consider this system of regulation to be operating under positive or negative control?