In which of the following conditions would a person not hear an echo?
a. a sound wave reaches the ear, and the reflected wave reaches the ear less than 0.1 seconds later.
b. the reflecting source is approximately 21.5 meters from the person.
c. a person is standing 30 meters in front of the reflecting surface.
d. a person is standing in the range of the sound waves coming from the original sound source as well as the reflected waves.

answer is: naturally occurring solid, inorganic compound or element is mineral.

minerals usually have crystalline form, that means than crystal constituents (atoms, molecules or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

for example magnetite is a mineral of iron oxide (fe₂o₃).

calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (caco₃).

dolomite is an   carbonate mineral composed of calcium magnesium carbonate (camg(co₃)₂).

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.