Two spherical mirrors, mirror A concave and mirror B convex, are made out of the same spherical ball of glass of radius 20 cm. Following New Cartesian sign convention, the focal lengths of A and B are ______ and _______, respectively.
1
-10 cm, -10 cm
2
10 cm, 10 cm
3
10 cm, -10 cm
4
-10 cm, 10 cm