The amount of oxygen in earth's atmosphere is

1. 60%
2. 21%
3. 78%
4. 10%

Earth's biosphere is also called

1. ecosystem
2. environment
3. atmosphere
4. hydrosphere

Earth completes its one revolution around the sun in

1. 360 days
2. 365 1⁄4 days
3. 366 days
4. 300 days

Earth's shape is described as

1. Circular
2. Spherical
3. Oblate spheroid
4. Oval