Let $A$ be the set of all continuous functions $f:\left [ 0,1 \right ]\rightarrow \left [ 0,\infty \right )$ satisfying the following condition:
$$\int_{0}^{x}f\left ( t \right )dt\geq f\left ( x \right ), \:for\:all \:x\in\left [ 0,1 \right ].$$
Then which of the following statements is true?
- $A$ has cardinality $1$.
- $A$ has cardinality $2$.
- $A$ is infinite.
- $A$ is empty.