Diferencia entre explicar y justificar

Need with this match the pronoun with the sentence it belongs to: )

Achemist mixes 4 liters of one acid solution with 10 liters of a 40% solution, resulting in a 30% acid solution. what is the value of x, the acid concentration of the first solution

Which verb form best completes this sentence? a paco no le gustan estas pinturas porque él no el arte de los siglos pasados. a. comprendes b. comprende c. comprendo d. comprenden