PM2.5是指大气中直径小于或等于2.5μm的颗粒物,含有大量有毒、有害物质,也可称可入肺颗粒物,将0.0000025用科学记数法表示为____.
“LED”节能光源材料是砷化镓(GaAs),其中Ga的化合价为+3价,则As的化合价为( )
下列说法正确的是( )
— . I believe even eating a little fast food is also harmful to my health .
His family are worried about him because they haven't __________ letters from him for a long time.