Answer:D. Compound.
Explanation:
A compound is defined as a pure substance: formed when two or more elements chemically combine to form bonds between their atoms.
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Answer is: the third reaction.
Elements in this chemical reaction do not change their oxidation number. Hydrogen has oxydation number +1, sulfur oxidation number is +6, oxygen has oxidation number -2 and barium has +2 on both sides of chemical reaction. In other reactions elements change their oxidation numbers.
Hypotonic solution will make the red blood cell grow in size if in contact.
