Answer:
b) Silicon and oxygen
Explanation:
A compound is a pure substance that contains two or more atoms chemically joined together. Sand is regarded as a 'compound' because it is made up of two or more atoms that are chemically bonded to one another.
Sand is said to predominantly contain silica, which has a chemical formula of SiO2. Hence, sand can be said to be a compound containing one atom of silicon (Si) and two atoms of oxygen (O2).
Electrons are usually the smallest unit in a compound.
Answer: First Law of Thermodynamics
Explanation:
According to first law of thermodynamics, energy of a closed system remains conserved and thus energy can neither be created nor be destroyed, it can only transform from one form to another.

= change in internal energy
q= heat absorbed or released
w= work done by or on the system
Thus change in energy is brought by work or heat which are also forms of energy. Thus energy gets transformed from one form to another and neither gets created or destroyed.