Before Jonas Salk Discovered a Vaccine to Prevent Polio,people Noticed

Before Jonas Salk discovered a vaccine to prevent polio,people noticed a correlation between outside temperature and the incidence of polio.Polio cases tended to occur more in the summer months than in the winter months,and the public assumed that high temperatures alone contributed to infection.As it turned out,there was a positive correlation between temperature and polio outbreaks because polio tended to be contracted in swimming pools and other places where children congregated.This illustrates which of the following? Correlations are

A) indicative of a causal relation.
B) variable.
C) invalid.
D) no guarantee of a causal relation.
E) unreliable.