Which of the following food chain shows spindle shaped pyramid of number?

• A

  Grass $\rightarrow$ Insects $\rightarrow$ Frog

• B

  Phytoplankton $\rightarrow$ Zooplankton $\rightarrow$ Small fish

• C

  Tree $\rightarrow$ Birds $\rightarrow$ Hawks

• D

  Tree $\rightarrow$ Birds $\rightarrow$ Insects $\rightarrow$ Bacteria