Two directly transmitted viral diseases have the same critical immunization threshold (p.) even though their effects on the host are different. Which of the following differences between the two diseases could lead to the same critical immunization threshold? a) One disease has a higher transmission rate and a longer recovery period than the other b) One disease has a lower transmission rate and a longer recovery period than the other c) One disease has a lower transmission rate and a shorter recovery period than the other. d) For one disease, the recovery period is long; for the other, no one recovers