Let $a$ and $b$ be non-zero real numbers. Then,the equation $(a x^2+b y^2+c)(x^2-5 x y+6 y^2)=0$ represents

• A
  four straight lines,when $c=0$ and $a, b$ are of the same sign
• B
  two straight lines and a circle,when $a=b$,and $c$ is of sign opposite to that of $a$
• C
  two straight lines and a hyperbola,when $a$ and $b$ are of the same sign and $c$ is of sign opposite to that of $a$
• D
  a circle and an ellipse,when $a$ and $b$ are of the same sign and $c$ is of sign opposite to that of $a$