If a line segement `A M=a`
moves in the plane `X O Y`
remaining parallel to `O X`
so that the left endpoint `A`
slides along the circle `x^2+y^2=a^2,`
then the locus of `Mdot`
A. `x^(2)+y^(2)=4a^(2)`
B. `x^(2)+y^(2)=2ax`
C. `x^(2)+y^(2)=2ay`
D. `x^(2)+y^(2)-2ax-2ay=0`
