Which of the following is a true statement about bacteria, based on what Tim and Moby explained?? A: Bacteria exist in only a few environments on Earth.
B: Bacteria can grow to extremely large sizes.
C: Mast bacteria are harmful to humans
D: Bacteria can be both harmful and helpful to humans.

Extra points for help

Ineed ! i have no clue how to do this problem.

An alloy contains 66 g of pure zinc. what is the percentage of zinc in the alloy? express your answer to two significant figures and include the appropriate units.

Astudent pours 10.0 g of salt into a container of water and observes the amount of time it takes for the salt to dissolve. she then repeats the process using the same amounts of salt and water but this time she slowly stirs the mixture while it is dissolving. the student performs the experiment one more time but this time she stirs the mixture rapidly. the dependent variable in this experiment is: time for salt to dissolve speed of stirring amount of water mass of salt