In a two-tailed test about the population mean, where the population standard deviation is unknown, a sample of size 40 was taken. Use 5% significance. Find the critical value(s). Select one: a. Only 1.685 b. Only 1.645 c. Both 2.023 and 2.023 d. Only 1.645 e. Both 1.96 and 1.96 f. Only 1.685