Помогите с ответом! Since ancient times Nature ___ Man, being the source of his life. a) is serving b) serves c) has served d) serve

Вопрос школьника по предмету Английский язык

Помогите с ответом!
Since ancient times Nature ___ Man, being the source of his life.
a) is serving
b) serves
c) has served
d) serve

Ответ учителя по предмету Английский язык

Since ancient times Nature ___ Man, being the source of his life.
c) has served