《1 认识二元一次方程组》巩固自测

下列四个方程组中，①$\left\{\begin{array}{l}x^{2}+3 y=4 \\ 3 x-5 y=1\end{array}\right.$②$\left\{\begin{array}{l}x y=1 \\ x+2 y=8\end{array}\right.$③$\left\{\begin{array}{l}a-b=3 \\ \frac{1}{a}-3 b=4\end{array}\right.$④$\left\{\begin{array}{l}a+3 b=4 \\ 7 a-9 b=5\end{array}\right.$二元一次方程组有个.