What is the rate of reaction?

Why does the rate of any reaction generally decrease during the course of the reaction?

The term $\left( -\frac{dc}{dt} \right)$ in a rate equation refers to the:

If the rate of disappearance of $N_2O_5$ in the following reaction is $1.2 \times 10^{-5} \ mol \ L^{-1} \ s^{-1}$,the rate of production of $NO_2$ in $mol \ L^{-1} \ s^{-1}$ is:
$2N_2O_{5(g)} \longrightarrow 4NO_{2(g)} + O_{2(g)}$

Thermal decomposition of $N_2O_5$ occurs as per the equation below:
$2N_2O_5 \longrightarrow 4NO_2 + O_2$
The correct statement is:

For a reaction,$2A + B \rightarrow 2C$,the rate of disappearance of $A$ is $0.076 \ mol \ dm^{-3} \ s^{-1}$. What is the rate of disappearance of $B$?