Questions

Mushrooms are heterotrophs. They are not plants or autotrophs.

Every living thing that exists need energy in order to live. This energy ultimately comes from the sun, and only plants, or autotrophs, can directly use this energy to make food. Those organisms that cannot harness the sun's energy are known as heterotrophs.

Mushrooms belong to kingdom Fungi and are heterotrophs because they decompose and consume nutrients from the soil. They are not green so they cannot photosynthesize

For this reason, mushrooms and all other fungi are heterotrophs.

‘Suppose’ and ‘if’ both are used to guess, or think, or imagine something and both means the same thing. So, using them together is superfluous. So, ‘if’will be used. 

For floating point numbers (float, double, long double) the values cannot be predicted exactly. Depending on the number of bytes, the precession with of the value represented varies. Float takes 4 bytes and long double takes 10 bytes. So float stores 0.9 with less precision than long double.

When atoi() tries to convert argv[0] to a number it cannot do so (argv[0] being a file name) and hence returns a zero.