Questions

Because of its being pollution free and  combustion efficient, Hydrogen gas is  called fuel of future. Because of its origin in marshy places, Methane is called as 'Marsh Gas'.

Natural gas is primarily composed of methane, but also contains ethane, propane, and heavier hydrocarbons.

Natural gas is a colorless, tasteless, odorless, and non-toxic gas. Because it is odorless, mercaptan is added to the natural gas, in very small amounts to give the gas a distinctive smell of rotten eggs. This strong smell can alert you of a potential gas leak.

It also contains small amounts of nitrogen, carbon dioxide, hydrogen sulphide and trace amounts of water.

The rule followed here is

7 + 3 - 4 = 6

Similarly,

4 + 2 - 2 = 4

9 + 5 - 4 = 10

Hence, the missing numbers are 4 & 10.

Why Water Is Not an Element :

An element is a substance consisting of only one type of atom. Water consists of two types of atoms: hydrogen and oxygen.

water is both a molecule and a compound :

A compound forms whenever two or more atoms form chemical bonds with each other. The chemical formula for water is H2O, which means each molecule of water consists of one oxygen atom chemically bonded to two hydrogen atoms.

Thus, water is a compound.

It's also a molecule, which is any chemical species formed by two or more atoms chemically bonded to each other. The terms molecule and compound mean the same thing and can be used interchangeably.

Hence, water is both a molecule and a compound.

The correct order of events during meiosis is:

prophase I,

metaphase I,

anaphase I,

telophase I,

cytokinesis, and

meiosis II.