State the most appropriate null and alternative hypotheses for each of the following experiments or observational studies:

a. A test of whether cigarette smoking causes lung cancer

b. An experiment to test whether mean herbivore damage to a genetically modified crop plant differs from that in the related unmodified crop

c. A test of whether industrial effluents from a factory into the Mississippi River are affecting fish densities downstream

d. A test of whether providing municipal safe-injection sites for drug addicts influences the rate of HIV transmission.