A group of similar (accoding to function) types of cells, when comes together to perform a specific function, then it forms a tissues.
When a group of similarly working tissues come together then they form an organ.
And this relation could be written as
cell > tissue > organ
So, organ is a collection of tissues and tissues are collection of cells.
Answer:
"theory of evolution by natural selection,”
this was proposed by charles darwin and alfred wallace in the 19th century
Since it is outside your body, the answer is B, external stimulus
