若计算(-2x+a)(x-1)的结果不含x的一次项,则a=______.
(-2x+a)(x-1)=-2x2+(a+2)x-a,
因为积中不含x的一次项,则a+2=0,
解得a=-2.
As there is less and less coal and oil, scientists are exploring new ways of making use of ________ energy, such as sunlight, wind and water for power and fuel. [ ]
A. primary
B. alternative
C. instant
D. unique
人类科学家把下列哪项作为人与猿分界的重要标志 [ ]
A.下肢增长
B.直立行走
C.骨盆增宽
D.脊柱变成“S”形