条形统计图是用（ ）表示数量的多少；折线统计图是用（ ）表示数量的多少，再用线段把相邻的各个点（ ）连接起来。
直条；点；顺次
For price discrimination to work, the seller must face a market with all of the following characteristics EXCEPT:（）
A. high barriers to entry.
B. a downward sloping demand curve.
C. two or more identifiable groups of customers with different price elasticities of demand for the product.
铸铁管水力计算表中流速的单位是（）。
A、m
B、mm
C、m／s
D、L／S
