Why could we not say 'an actual prototype' instead of the given answer?

[Borgisson]

Business English Lesson, Advanced Level

ESL/EFL Test #168 "Product Development (2)", question 4

The product and process design stage involves turning the concept into a tangible product design and transforming that design into ......... prototype or pilot.

(a) an active
(b) an actual
(c) a working
(d) a solid

Business English Lesson, Advanced Level

ESL/EFL Test #168 "Product Development (2)", answer 4

The product and process design stage involves turning the concept into a tangible product design and transforming that design into a working prototype or pilot.

Correct answer: (c) a working

Your answer was: incorrect
The product and process design stage involves turning the concept into a tangible product design and transforming that design into an actual prototype or pilot.
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Dear Sir/Madame

Why could we not say "an actual prototype" instead of the given answer?

Thank you for your time

Borgisson

[Mister Micawber]

.
'Actual prototype' is unrealistic here. There was no chance for a 'pseudo-prototype'.
.
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Native English teacher at Mister Micawber's