And this one too, i really need help

In which situation can a mixture always be called a solution

Nebulae are enormous clouds in outer space. they are made mostly of hydrogen gas, helium gas, and dust. some nebulae glow brightly, while others do not. the stars that people see are huge, bright balls of glowing gas. they are made mostly of hydrogen and helium. which statement correctly describes other ways in which nebulae and stars are different? a. stars can form inside a nebula but a nebula can never be produced by any star. b. a star always has a higher density than a nebula. c. stars can never form inside a nebula but a nebula can be produced by any star. d. a nebula always has a higher density than a star.

Twenty-seven milliliters of an acid with an unknown concentration are titrated with a base that has a concentration of 0.55 m. the indicator changed color when 12.5 milliliters of base were added. what is the concentration of the unknown acid?