In a certain sequence of numbers, a1, a2, a3, ..., an, the average (arithmetic mean) of the first m consecutive terms starting with a1 is m, for any positive integer m. If a1=1, what is a11?	Choose the correct choice
A) 100
B) 55
C) 21
D) 19
E) 1
  As stated above, (a1+a2+a3......+a10+a11)/11=11   therefore a1+a2+a3.......a11=121 (1)   using the same logic, we got a1+a2+a3..........+a9+a10=100 (2)   (2)-(1) we got a11=21   21
<result>C</result>
In a certain sequence of numbers, a1, a2, a3, ..., an, the average (arithmetic mean) of the first m consecutive terms starting with a1 is m, for any positive integer m. If a1=1, what is a11?