平面内有两个定点F1,F2和一动点M,设命题甲,||MF1|﹣|MF2||是定值,命题乙:点M的轨迹是双曲线,则命题甲是命题乙的( )
A:充分但不必要条件
B:必要不充分条件
C:充要条件
D:既不充分也不必要条件
Those who think that sharks are _____ strong _____ be endangered are wrong.
delicious, studying, tomorrow, use, house
有关地震波的说法,正确的是( )