设随机变量(X,Y)服从二维正态分布N(0,0;1,4;-1/2),随机变量中服从标准正态分布且与X独立的是()
C
当x→0时，与根号x等价的无穷小量是（）
How long ________ they ________ (dig) the ditch?
Be careful, _______ you'll fall down.
新民主主义社会是 _______。
he teacher told them since light ________ (travel) faster than sound,lightning ________ (appear) to go before thunder.