Monomers bind chemically to other molecules to form polymers
 
        
             
        
        
        
20. The answer is 1 (Precambrian)
21. The answer is 4 (Pennsylvanian)
22. The answer is 2 (Bony Fishes, Dinosaurs, and Mammals)
23. The answer is 2 (Reptiles)
        
             
        
        
        
Answer: Option A) receiving; sending
Explanation:
The neuron is made up of many parts, they include:
- dendrites, which are branched and helps in receiving impulses from the adjacent neuron 
- axon, a long and extended portion of the neuron that the transmission of impulses along its length away from the cell body.
- synapse etc
Thus, dendrites of neurons is involved in receiving impulses into the neuron, while axons is involved in sending impulses away from the neurons