2. “Real numbers $a, b, c, d$ form an arithmetic sequence in order” is a ( ) condition for “$a+d=b+c$”.
(A) Sufficient but not necessary
(B) Necessary but not sufficient
(C) Sufficient and necessary
(D) Neither sufficient nor necessary
2. A.

Sufficiency is obvious.
Necessity: a counterexample is $3+1=2+2$.