What day is considered to be the first day of a female's reproductive cycle? when hormone levels are low and the uterus lining is shed. when hormone levels are high and the uterus lining is shed. when hormone levels are low and the ovary releases an egg. when hormone levels are high and the ovary releases an egg.

The answer is atmosphere. there are four spheres of the earth: 1. geosphere (from greek word 'geo' = ground), 2. hydrosphere (from greek word 'hydro' = water), 3. biosphere (from greek word 'bio' = life), and 4. atmosphere (from greek word 'atmo' = air). a magnetosphere is a region of space around astronomical objects controlled by the magnetic field of that object. through the process of elimination, since lightning is present in the air, it can be found in the atmosphere.

Sand and gravel would be the answer