Questões de Concurso Público Prefeitura de São Borja - RS 2019 para Professor de Português/Inglês

Consider the use of the apostrophe in the expressions “speakers' first language” (title) and “speaker's first language” (l.05-06):
I. In the first token, it is added to make the noun plural. II. In the second token, it indicates that there is an omission of the verb to be. III. It is wrongly used in the former, as it should have been written as in the latter.
Which ones are INCORRECT?