1. 下列方程组中不是二元一次方程组的是 ()
A. $\left\{\begin{array}{l} x= 2,\\ y= 3\end{array}\right. $
B. $\left\{\begin{array}{l} x+y= 1,\\ x-y= 2\end{array}\right. $
C. $\left\{\begin{array}{l} x+y= 5,\\ xy= 1\end{array}\right. $
D. $\left\{\begin{array}{l} y= x,\\ x-2y= 1\end{array}\right. $

C

2. 已知$x= 4,y= -2与x= -2,y= -5都是方程y= kx+b$的解,则k与b的值分别为 ()
A. $k= \frac {1}{2},b= 4$
B. $k= \frac {1}{2},b= -4$
C. $k= -\frac {1}{2},b= 4$
D. $k= -\frac {1}{2},b= -4$

B

3. [2024·安阳一模]明代数学著作《算法统宗》一书中记载这样一题:“隔墙听得客分银,不知人数不知银,七两分之多四两,九两分之少半斤(一斤= 16两).问:人和银各几何?”其大意:隔墙听人分银子,每人分7两,则多4两;每人分9两,则少半斤.问人和银各多少.设共有x人,y两银,则可列方程组为 ()
A. $\left\{\begin{array}{l} 7x-4= y,\\ 9x-8= y\end{array}\right. $
B. $\left\{\begin{array}{l} 7x+4= y,\\ 9x-8= y\end{array}\right. $
C. $\left\{\begin{array}{l} 7x-4= y,\\ 9x+8= y\end{array}\right. $
D. $\left\{\begin{array}{l} 7x+4= y,\\ 9x+8= y\end{array}\right. $

B