What type of molecule speeds up chemical reactions in living things?

a. lipid
b. nucleic acid
c. protein
d. water

The best answer is  B - Proteins.

Proteins make up enzymes, or to put it another way enzymes are made up of protein. It is enzymes that speed up the chemical reactions. Enzymes are organic catalysts, substances that speed up a (biochemical) reaction but do not take part in the reaction and remain unchanged at the end of the reaction.

In man, there are many enzymes. The majority of these are found in the digestive system where they greatly aid the process of food breakdown from complex molecules to simple molecules. Examples include salivary amylase found in saliva,  which begins the gradual breaking down of starch into glucose right from the moment food is placed in the mouth and mixed with saliva. Pepsin in the stomach breaks down protein.

after receiving a stimus, the sensory receptors send(s) signals to the appropriate area of the body.  

the must be sensory neurons.  

hope its .

sulfur dioxide

explanation: acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. these substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.