Study guide, I want to make sure I got this question right

(6) A high school has 1,000 students: the numbers of students in 9th, 10th, 11th and 12th grades are 400, 300, 200 and 100 respectively. Randomly sample 20 students with replacement. Let Nk denote the number of kth graders in the sample, k = 9, 10, 11, 12. Circle the correct answer in each of parts (a), (b), (c); and show your work in part (d). (a) The probability distribution of Nil is (1) Bin(20, 0.2); (ii) Bin(4, 0.2); iii Bin (20, 1/4). (b) The probability distribution of No + N10 is (1) Bin(4, 0.7); (ii) Bin(20, 0.7); (iii ) Bin (20, 0.5). (c) The correlation between Ng and N12 is -4/5, 3V6' -1. (d) Calculate the covariance between Ng + Nio and Nil + N12