Which type of data $($cross$-$sectional or time series$)$ is each variable?

a$)$ scores of $50$ students on a midterm accounting exam last semester

b$)$ bob's scores on $10$ weekly accounting quizzes last semester

c$)$ average score by all takers of the states CPA exam for each of the last $10$ years

d$)$ number of years of accounting work experience for each of the $15$ partners in a CPA firm