"A variety" can take a singular or plural verb depending on the context and the emphasis. For this question, the answer could be either singular or plural. The good news is that the SAT won't test you on this distinction. Chapter 15 Tenses, Exercise 3, #3 (pg. 83): Answers A and B are both possible. This question needs to be revised to be more ... Answers 312 Index 342 Chapter 10 Chapter 11 Chapter 12 Permutations and combinations 123 Factorials 124 Permutations 129 Combinations 130 The binomial coefficients 132 Using binomial coefficients to calculate probabilities 133 The binomial distribution 141 The binomial distribution 143 The expectation and variance of B(n, p) 146