Clinical literature reports that the duration of a typical cold is roughly 18 days. Researchers believe that people tend to underestimate the duration of a typical cold, on average. To determine if people do indeed underestimate the duration of a typical cold, the researchers planned to survey a random sample of 400 healthy adults in California and asked them how long they think a typical cold lasts.

What are the null and alternative hypotheses that these researchers wanted to test?

a. H_o: =18 H_a: < 18

b. H_o: =18 H_a: > 18

c. H_o: =18 H_a: 18

d. H_o: x =18 H_a: x < 18