The correct answer is D.
Echo refers to the sound that is heard when a sound wave is reflected back from a surface. An echo sound is made when a sound wave traveled through the air, hit a hard surface and get reflected back. Generally, hard surfaces reflect sound wave while soft surfaces absorb sound waves. Reflected sound waves are used in application of some devices that are used in range and direction finding.
Solid (ice caps)
Liquid (oceans, rivers, lakes, etc)
Gas (clouds)
The circulatory system works with the digestive system. Once the food is digested, the circulatory systems absorb and uses the nutrients in the food. If the digestive system were to break down, the circulatory will not have the nutrients it needs to sufficiently run the body.
A conjugate acid is a conjugate base with hydrogen ions attached to it. In this case, the conjugate base is the carbonate ion, CO₃⁻². This ion can have two hydrogen ions, so the conjugate acid is:
H₂CO₃
This compound is known as carbonic acid.
I believe there are 3 significant figures B.