1. The locus of the center of a circle that is tangent to both circles $x^{2}+y^{2}=1$ and $x^{2}+y^{2}-8 x+7=0$ is
A. Two ellipses
B. Two hyperbolas
(․) One hyperbola and one line
D. One ellipse and one hyperbola
1. C Hint: If it is externally tangent to two given circles, then it is a hyperbola; if internally tangent, then it is a straight line.