An internet service provider (ISP) provides internet connections to 100,000 customers. 10,000 of the customers have high-speed connections and 90,000 of the customers have low-speed connections. The ISP wants to know whether, on the average, customers who have high-speed connections use email more frequently than customers who have low-speed connections. To find out, the ISP takes a simple random sample of 150 high-speed-connection customers and an independent random sample of 300 low-speed-connection customers. For each customer in the sample, they find the number of email messages sent and received in the previous month. Then they compute the sample mean and sample standard deviation of each of the two sets of numbers. Let h denote the average number of emails sent and received in the previous month among all customers with high-speed connections, and let l denote the average number of emails sent and received in the previous month among all customers with low-speed connections. Let H and L be the corresponding sample means. Let Sh be the sample standard deviation of the number of emails sent and received in the previous month by customers with high-speed connections, and let Sl be the sample standard deviation of the number of emails sent and received in the previous month by customers with low-speed connections.

Suppose that the population distributions of the number of emails sent in the by high-speed-connection customers and by low-speed-connection customers both are nearly normal. Which of the following have probability histograms that can be approximated well by a normal curve, after transforming to standard units?

a. L
b. H-L
c. H
d. I
e. h - l
f. h