Acompany is interested in monitoring the average time it takes to serve its customers. an appropriate control chart would be:

X-bar chart

Explanation:

In industrial statistics, the X-bar chart is a type of Shewhart control chart that is used to monitor the arithmetic means of successive samples of constant size, n. The X-bar chart shows how the mean or average changes over time .For example, one might take a sample of 5 shafts from production every hour, measure the diameter of each, and then plot, for each sample, the average of the five diameter values on the chart.  For the purposes of control limit calculation, the sample means are assumed to be normally distributed, an assumption justified by the Central Limit Theorem.

d. low interest rates