Bought up vs. sold off

[Paul]

Test No. incompl/inter-1 "Applying for that Job", question 7

You should have ......... those shares when they were cheap.

(a) taken out
(b) sold off
(c) bought up
(d) taken over

Test No. incompl/inter-1 "Applying for that Job", answer 7

You should have bought up those shares when they were cheap.

Correct answer: (c) bought up

Your answer was: incorrect
You should have sold off those shares when they were cheap.
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Dear teacher,
why can't we use "sold off" in this sentence?
Thank you

[Alan]

You buy shares when they are cheap and sell them later when the value has gone up. You don't sell them when they are cheap or you will lose money.
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English as a Second Language
You can read my ESL story A day in the life of a driving instructor