Probability Concepts Which of these numbers cannot be a probability?

a) $-0.00001$

b) $0.5$

c) $1.001$

d) $0$

e) $1$

f) $20\%$

Probability can either be expressed in percentages, decimals, proper fractions or numbers between 0 to 1, that is, not likely to happen or most likely to happen. Therefore a and c are not probability. What are the concepts behind this? Thanks in advance!
(Thank you. I found the defining properties of probability which sums up your comments)

Probability has two defining properties:

1.The probability of any event is a number between 0 and 1, or 0 &lt; P(E) &lt; 1. A P followed by parentheses is the probability of (event E) occurring. Probabilities fall on a scale between 0, or 0%, (impossible) and 1, or 100%, (certain). There is no such thing as a negative probability (less than impossible) or a probability greater than 1 (more certain than certain).

2.The sum of all probabilities of all events equals 1, provided the events are both mutually exclusive and exhaustive. If events are not mutually exclusive, the probabilities would add up to a number greater than 1, and if they were not exhaustive, the sum of probabilities would be less than 1.
You have just given the answer! Probability can be $0$ between it and $1$ or $1$. So a) and c) are the answers. 
Laws in probability state that sum of all probability equals $1$.