Cloud cover is the percent of Earth's surface covered by clouds! Cloud cover may increase due to an increase in cosmic ray flux (the number of high-energy particles from space that reaches Earth). Figure 1 shows the relationship between low cloud cover and relative cosmic ray flux (RCRF). What was the monthly average cloud cover in January of 1990?

a. 20.1%
b. 7%
c. 19.25%
d. 3.5%