Doctors have fought infections with penicillin for over 50 years. recently, penicillin has not worked as well as before. which of the following explanations can account for this observation? a. some bacteria are resistant to the drug. the continued use of the drug has killed all of these bacteria. b. penicillin use has made all bacteria resistant to it. c. some bacteria are resistant to penicillin. the continued use of the drug has killed all but these bacteria. d. bacteria eat penicillin, so it is not effective anymore.

C. "A" mentions that the drug killed all of the bacteria that are resistant to the drug, and that doesn't make any sense. "B" claims that all bacteria are resistant to the drug. This is not true. "D" mentions that bacteria eat the drug, which doesn't happen.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

a boiling water reactor does not boil the water. a pressurized water reactor boils the water faster. a pressurized water reactor is heated directly by the core. a pressurized water reactor does not use fission.