The cause of variation is hidden in the process of $.....P.....$ - this knowledge was acquired by humans in $.....Q.....$.
$\quad \quad P \quad\quad\quad Q$

• A
  Sexual reproduction $\quad\quad 8000-1000 \text{ B.C.}$
• B
  Asexual reproduction $\quad\quad 8000-1000 \text{ B.C.}$
• C
  Sexual reproduction $\quad\quad 8000-10,000 \text{ B.C.}$
• D
  Asexual reproduction $\quad\quad 8000-10,000 \text{ B.C.}$