Why not 'above board'?

[Priya Raghavan]

English Language Tests, Intermediate level

ESL/EFL Test #299 "English Slang Idioms (21)", question 1

He was sick of working for companies that weren't above ......... about their business practices. Too many of them had cut illegal and unethical corners.

(a) board
(b) grade
(c) mediocre
(d) ideal

English Language Tests, Intermediate level

ESL/EFL Test #299 "English Slang Idioms (21)", answer 1

He was sick of working for companies that weren't above grade about their business practices. Too many of them had cut illegal and unethical corners.

Correct answer: (b) grade

Your answer was: incorrect
He was sick of working for companies that weren't above board about their business practices. Too many of them had cut illegal and unethical corners.
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Why not 'above board'?

Priya Raghavan

[Mister Micawber]

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'Above board' is the correct answer. The answer key is faulty, and one of the administrators will soon fix it. Thank you for bringing this to our attention.
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Native English teacher at Mister Micawber's