Select the correct statements regarding the characteristics of acquired immunity.
1. Cell-mediated immunity is responsible for acquired immunity.
2. It produces a primary response of low intensity.
3. Active and passive immunity are types of acquired immunity.
4. Polymorphonuclear leucocytes and natural killer cells are involved in acquired immunity.

A. 1, 2 and 3

B. 1, 3 and 4

C. 1 and 4

D. 1 and 3

Answer: Option A