泵使用变频技术后，实质上是改变泵的特性曲线而没有改变（）曲线。
A、温度特性
B、管路特性
C、电流特性
D、功率特性
参考答案：B
Selected financial data from Krandall, Inc. ' s balance sheet for the year ended December 31 was as follows (in $ ) :
A．Cash
B．$1100000
C．Accounts Payable
D．$ 400000
E．Accounts Receivable
F．300000
G．Deferred Tax Liability
H．700000
I．Inventory
J．2400000
K．Long-term Debt
L．
8200000
M．Property, Plant & Eq.
N．8000000
O．Conunon Stock
P．1000000
Q．Total Assets
R．11800000
S．Retained Earnings
T．1500000
U．LIFO Reserve at Jan. 1
V．600000
W．Total Liabilities & Equity
X．11800000
Y．LIFO Reserve at Dec. 31
Z．900000
Z．
“前四史”中成书时间最晚的是（）
A、《史记》
B、《汉书》
C、《后汉书》
D、《三国志》
