题目：In a numerical table with 1 0 rows and 1 0 columns,each e ntry is either a 9 or a 1 0 . If the number of 9s in the nth row is n - 1 for each n from 1 to 1 0 , what is the average (arithmetic mean) of all the numbers in the table?
A
9.45
B
9.50
C
9.55
D
9.65
E9.70
答案：C
解析：There are (10)(10) entries in the table. In rows1,2,3 .... ，10，the number of 9s is 0,1,2....,9, respectively, giving a total of 0+1+2...+9=45 entries with a 9. This leaves a total of 100-45=55 entries with a 10. Therefore, the sum of the 100 entries is 45(9)+55(10)=405+550=955, and the average of the 100 entries is 955/100=9.55