Task: Question Answering
Input (English): Problem: the probability of two events a and b are 0.20 and 0.40 respectively. the probability that both a and b occur is 0.15. the probability that neither a nor b occur is _________
Options: a. 0.45, b. 0.4, c. 0.5, d. 0.55, e. 0.6
Output (English): d