All of the following statements concerning Waiver of Premium riders are correct EXCEPT:
a) Once activated, the Waiver of Premium will continue until the insured's recovery or the maturity of the policy whatever occurs first
b) An insured who has recovered from a disabling injury will be required to repay the insurer for any premiums that were waived
c) Waiver of Premium riders require that disablement needs to last for a certain period of time
d) There is a maximum age limit for the Waiver of Premium rider to activate.

[120 total points] (public health) in one of the largest human experiments ever conducted, in 1954 a randomized controlled trial was run to see whether a vaccine developed by a doctor named jonas salk was effective in preventing paralytic polio. a total of 401,974 children (ages 6–9), chosen to be representative of those who might be susceptible to the disease, were randomized to two groups: 200,745 children (the control group c) were injected with a harmless saline solution (a placebo) and the other 201,229 children (the treatment group t) were injected with salk’s vaccine. what was the point of giving saline solution to the children who didn’t get the vaccine? explain briefly.