Choose the alternative which best expresses the meaning of the highlighted word: "$I$ cannot $conceive$ of a time when $I$ was without a refrigerator."

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Though he has a lot of money,yet all his plans are $built\, upon\, sand.$

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Despite the trust bestowed on the minister, he turned out to be $a\, snake\, in\, the\, grass$ during the revolution.

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You have to be $a\, cool\, customer$ and be patient if you want to get the best buys.

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His friends $let$ him down.

Some idioms/phrases are given below together with their meanings. Choose the correct meaning of the idiom/phrase: "To be above board".