If in a population of size $N$,the birth rates are represented as $b$ and death rates as $d$,then the increase or decrease in $N$ during a unit time period $t$ will be:

• A
  $\frac{dN}{dt} = (b - d) \times N$
• B
  $\frac{dN}{dt} = (d - b) \times N$
• C
  $\frac{dN}{dt} = (b - N) \times d$
• D
  $\frac{dN}{dt} = (N - d) \times b$