A satellite is in a $7000-\mathrm{km}$ -radius circular orbit around Earth. If the satellite moves to a new orbit with period twice that of the original, the new radius is
(a) $10,000 \mathrm{~km}$
(b) $11,100 \mathrm{~km}$;
(c) $14,000 \mathrm{~km}$
(d) $19,800 \mathrm{~km}$