Why is $s_p$ called the pooled sample standard deviation?
A. Because it is obtained by combining information on variation from two distinct population standard deviations to get a single new population standard deviation
B. Because it is obtained by combining information on variation from the individual samples into one estimate of the common sample standard deviation
C. Because it is obtained by combining information on variation from the individual samples into one estimate of the common population standard deviation
D. None of the above