【Analysis】He ran a total of 5 hours in a month, indicating there were 5 Mondays, Saturdays, and Sundays. $31 \div 7=4$ weeks $\cdots 3$ days, which means the 1st of the month was a Saturday, so the 8th was a Saturday, and the 10th was a Monday.

【Solution】Solution: He ran a total of 5 hours in a month, indicating there were 5 Mondays, Saturdays, and Sundays. $31 \div 7=4$ weeks $\cdots 3$ days,

which means the 1st of the month was a Saturday,
so the 8th was a Saturday, and the 10th was a Monday.
Therefore, the answer is: D.
6. (10 points) Teacher Zhang runs for 20 minutes every Monday, Saturday, and Sunday, and jumps rope for 20 minutes every other day. In a certain month, he runs a total of 5 hours. Therefore, the 10th day of this month is ( )
A. Sunday
B. Saturday
C. Tuesday
D. Monday