3. Given condition $p: a, b, c, d$ form a geometric sequence; condition $q: ad=bc$. Then condition $p$ is a (.) condition of condition $q$.
(A) Sufficient but not necessary
(B) Necessary but not sufficient
(C) Sufficient and necessary
(D) Neither sufficient nor necessary
3. A.

Sufficiency obviously holds, necessity does not hold.
For example, $a=1, b=2, c=5, d=10$.