A researcher wants to prove that there is a difference in the average life spans between men and women in Japan. Let mu1 = average life span of Japanese women and mu2 = average life span of Japanese men. A random sample of 10 women showed an average lifespan of 83 years, with a sample standard deviation of 7 years. A random sample of 10 men showed an average lifespan of 77 years, with a sample standard deviation of 6.4 years. Assume that life spans are normally distributed and that the population variances are equal. What is the pooled estimate of the variance for this problem?

A.[(9)(7) + (9)(6.4)]/20

B.[(9)(49) + (9)(40.96)]/20

C.[(9)(49) + (9)(40.96)]/18

D.(7+6.4)/20