Rex is currently taking physics as one of his electives in school. His grade at the end of the year is determined by the average of four exams, each worth 100 points. On his first test he got an 84, on his second he got a 86, and on his third an 92. What must he score on his fourth exam to get an A (90%) in the course?

a. 98

b. 90

c. 88

d. 86