# If $a_1, a_2, ...., a_n$ are in HP, then the expression $a_1a_2 + a_2a_3 + .... + a_{n-1}a_n$ is equal to :
( A ) $n (a_1 - a_n)$
( B ) $(n-1)(a_1 - a_n)$
( C ) $(n-1) a_1a_n$
( D ) $na_1a_n$