Test and analysis: from the stem leaf diagram,

The average value of a is,

The average value of b is,

The average value of b is greater than that of a,

As can be seen from the stem leaf diagram, the performance of B is relatively stable, so C is chosen.

Comments: This question examines the average and stability of two groups of data, which is a frequent problem. For two sets of data, we usually compare their average and stability, and pay attention to careful calculation.