1. We have seasons on Earth because
(a) the orbit of the Earth is not exactly circular
(b) Earth is closer to the Sun in summer
(c) Earth rotates on its axis
(d) Earth's axis of rotation is tilted with respect to the ecliptic
(e) Earth's orbit oscillates around the celestial sphere

2. Retrograde motion of planets like Mars proved easy to explain once
astronomers realized that 
(a) the Earth was round
(b) the planets orbited the Sun
(c) the Earth's rotation axis is tilted
(d) the celestial sphere was not real
(e) the year is not exactly 365 days long

3. A perfect thermal radiator (blackbody) at a temperature of 6000K emits 
more radiation than a perfect blackbody at 5000K
(a) only at Ultra-Violet wavelengths
(b) only at optical wavelengths
(c) only at visible wavelengths
(d) at all wavelengths
(e) no radiation is emitted by perfect thermal radiators

4. X-ray telescopes have to be above Earth's atmosphere because
(a) it increases telescope's light gathering power
(b) it makes them closer to the distant stars
(c) Earth's atmosphere blocks X-ray radiation form space
(d) X-ray telescopes placed on the ground have very poor resolution
(e) X-ray radiation is harmful to humans