Platypus females lay their eggs in a burrow, and examples of monotremes are the Platypus and Echidna. Given these facts, what is the distinguishing feature of monotremes among the following options:
- They lay eggs
 - the eyes
 - Veins and arteries.
 - eardrum
 - four limbs
 - layers of fat
 - They have thick fur
 - fur and fat


 

 They lay eggs        