6000多年前半坡氏族家庭所从事的生产活动中， 物质发生了化学变化的是[ ]
A．搭建房屋
B．磨制石器
C．用麻织布
D．烧制瓷器
答案：D
He spends as much time as he can _______ English every day. [ ]
A. to practice speaking
B. to practice to speak
C. practice to speak
D. practicing speaking
采用指数平滑法进行预测时，如果时间数列变化剧烈，应当选择较大的平滑系数。
