The temperature at a mountain base camp was −4 degrees Celsius on Saturday. Sunday morning, the temperature was 2 degrees Celsius lower than what it was on Saturday. By Sunday evening, the temperature was 1 degree Celsius lower than what it was that morning. What was the temperature at the base camp Sunday evening?
a. −9 degrees Celsius
b. −8 degrees Celsius
c. −7 degrees Celsius
d. −5 degrees Celsius