4. Given three planes $\alpha, \beta, \gamma$, the angle between each pair of planes is $\theta$, and $\alpha \cap \beta=a, \beta \cap \gamma=$ $b, \gamma \cap \alpha=c$. If proposition 甲: $\theta>\frac{\pi}{3}$; proposition 乙: $a, b, c$ intersect at one point. Then
A. 甲 is a sufficient condition for 乙 but not necessary
B. 甲 is a necessary condition for 乙 but not sufficient
C. 甲 is a necessary and sufficient condition for 乙
D. None of A, B, C is correct