6 Given three different planes $\alpha, \beta, \gamma$ and two non-coincident lines $m, n$, there are the following 4 propositions:
(1) $m / / \alpha, \alpha \cap \beta=n$, then $m / / n$
(2) $m \perp \alpha, m / / n, n \subset \beta$, then $\alpha \perp \beta$
(3) $\alpha \perp \beta, \gamma \perp \beta$, then $\alpha / / \gamma$
(4) $\alpha \cap \beta=m, m \perp \gamma$, then $\alpha \perp \gamma$ The number of correct propositions is ( ).
(A) 2
(B) 3
(C) 4
(D) 1
$6 \mathrm{~A}$