The sequence a1, a2, ..., an, ... is such that an = (an-1 + an+1)/2 for all n > 1. If a2 - a1 = 2, then a10 - a1 =

This choice implies that the difference between the first and tenth term is only 9. However, since the sequence is arithmetic and each term increases linearly based on the common difference, this value does not accurately reflect the progression derived from the information given.

Selecting 10 as the difference suggests a very small common difference. Given that a2 - a1 = 2, this would not allow sufficient accumulation to reach a difference of 10 by the time we get to a10, indicating an incorrect calculation of the arithmetic progression.

This option implies a difference of 17, which is still not aligned with the arithmetic nature of the sequence. The correctly derived common difference from a2 and a1 leads to values that do not allow for a difference of 17 when calculating a10 - a1.

Choosing 19 as the difference suggests that the terms have progressed beyond what is possible given the established common difference. The arithmetic progression structure indicates that this value is unattainable given the initial condition of a2 - a1 = 2.