My favorite thing is aquaria. a place in which aquatic plants and animals are kept. The term is applied to single tanks for home use in which fish are kept for their decorative effect and interesting hobbies. It is also for public Institutions to conduct an exhibition and scientific study of aquatic life. i make home aquaria to stimulate a natural environment. Aquatic plants supply the oxygen needed by fish. but often an aerating device is used to furnish additional oxygen. Goldfish enjoy cool water, but tropical fish must have water at a constant temperature of 22°C or more. I must keep the water pure and free of algae by sufficient aeration and by keeping mollusks in the aquaria. A layer of sand or gravel at the bottom of the tank will hold the roots of such important aquatic plants and aid in the removal of debris.10. "... but tropical fish must have water at a constant temperature of 22°C or more." (Paragraph 2) The underlined word means
a. happening a lot or all the time
b. happening most of the time
C. happening some time
d. happening in high level​