下面直接反映了地球大气污染严重的是()
A:雾霾频繁出现
B:森林资源不断减少
C:干旱频繁发生
D:人畜饮用水短缺
请用上述物质的数字序号填空:
Do you have a baseball bat?
Yes,____ ____ .
Does Ann have a baseball?
No, ____ ____ .
Do Jack and Jim have computers?
Yes, ____ ____ .
Does Chi Yong have a soccer ball?
No,____ ____ .
Do you have a tennis racket?
No,____ ____ .
在三棱锥P-ABC中,PA=PB=PC,底面△ABC是正三角形,M、N分别是侧棱PB、PC的中点.若平面AMN⊥平面PBC,则平面AMN与平面ABC所成二面角(锐角)的余弦值等于( )