Fill in the blanks with suitable option: A/An ________ turns blue litmus red and a/an ________ turns ________ litmus blue.

Litmus is a natural indicator (from lichens) and is widely used for quick acid-base identification. Litmus shows its main colour-change range roughly around pH ~4.5 (red) to ~8.2 (blue). An acid turns blue litmus red, and a base turns red litmus blue.