1. 下列方程组中,是三元一次方程组的是()
A. $\left\{\begin{array}{l} x+z= 2,\\ xy+x= 4,\\ z-x= 1\end{array} \right. $
B. $\left\{\begin{array}{l} x-\frac {3}{y}= 4,\\ x+z= 6,\\ y-2z= 7\end{array} \right. $
C. $\left\{\begin{array}{l} x+z= 9,\\ x-y= 4,\\ z-y= 5\end{array} \right. $
D. $\left\{\begin{array}{l} x+y= 8,\\ y-m= 3,\\ z-x= 5\end{array} \right. $

C

2. 三元一次方程组$\left\{\begin{array}{l} x+y= 3,\\ y+z= 5,\\ x+z= 4\end{array} \right. $的解是()
A. $\left\{\begin{array}{l} x= 1,\\ y= 3,\\ z= 2\end{array} \right. $
B. $\left\{\begin{array}{l} x= 2,\\ y= 1,\\ z= 3\end{array} \right. $
C. $\left\{\begin{array}{l} x= 3,\\ y= 2,\\ z= 1\end{array} \right. $
D. $\left\{\begin{array}{l} x= 1,\\ y= 2,\\ z= 3\end{array} \right. $

D

3. 解方程组$\left\{\begin{array}{l} 3x-y+2z= 3,\\ 2x+y-4z= 11,\\ 7x+y-5z= 1.\end{array} \right. $若要使运算简便,消元的方法应()
A. 先消去$x$
B. 先消去$y$
C. 先消去$z$
D. 以上说法都不对

B

4. 某次足球联赛每支球队都要赛11场,已知胜一场记3分,平一场记1分,负一场记0分.某省队所负场数是所胜场数的$\frac {1}{2}$,结果共得20分,则该省队共平几场?若设该省队共胜$x$场,平$y$场,负$z$场,则所列方程组是()
A. $\left\{\begin{array}{l} x+y+z= 11,\\ 3y+x= 20,\\ \frac {x}{2}= z\end{array} \right. $
B. $\left\{\begin{array}{l} x+y+z= 11,\\ 3y+x= 20,\\ \frac {z}{2}= x\end{array} \right. $
C. $\left\{\begin{array}{l} x+y+z= 11,\\ 3x+y= 20,\\ \frac {z}{2}= x\end{array} \right. $
D. $\left\{\begin{array}{l} x+y+z= 11,\\ 3x+y= 20,\\ \frac {x}{2}= z\end{array} \right. $

D