Answer:
As the Earth revolves about the sun, different sky is visible during time of the year. We have winter constellations and summer constellations. But some constellations appear through out the year like Ursa Minor are the ones which lie close to the poles. The Earth rotates about the axis passing through North and south pole. The axis always points towards North. Thus, the constellations near the Poles would be visible throughout the year.
b. 30 to 60 years
c. 60 to 120 years
d. 150 to 500 years