"native speaker" vs "natural speaker"

[Miron]

English Language Tests, Intermediate level

ESL/EFL Test #98 "Student Teacher Dialogue (1)", question 6

Student: Just because you are a ......... speaker.

(a) natural
(b) native
(c) national
(d) nature

English Language Tests, Intermediate level

ESL/EFL Test #98 "Student Teacher Dialogue (1)", answer 6

Student: Just because you are a native speaker.

Correct answer: (b) native

Your answer was: incorrect
Student: Just because you are a natural speaker.
_________________________

why is it wrong?

Miron

[Torsten]

Because the correct phrase is "native speaker".
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