uppose you plan to perform a two sample z-test (or log rank test) to compare the Covid-19 outcome between the vaccinated and unvaccinated groups. What is the null hypothesis for this test in terms of the survival curves for the randomization groups (S_vaccine(t) and S_placebo(t))?

a. Svaccine(t)Splacebo(t)()()

b. Svaccine(t)=Splacebo(t()=()

c. Svaccine(t)Splacebo(t)0()()0

d. Svaccine(t)Splacebo(t)()()