[填空题]
(a) A brass plug is to wizg4l0p,ko g*.a yyo;yc6r*:x h7:s gpr-nvi: l*ibbe placed in a ring made of iron. At 20 $^{\circ} $ C, the diameter of the plug is 8.753 cm and that of the inside of the ring is 8.743 cm. They must both be brought to what common temperature in order to fit? $^{\circ} $ C
(b) What if the plug were iron and the ring brass? $^{\circ} $ C