the new lamp vs. a new lamp

[Suneethaago]

English Grammar Tests, Elementary Level

ESL/EFL Test #222 "Articles (3)", question 7

I have to buy ......... new lamp for the bedroom. I will go to the store on Saturday to find one that matches the other colors in the room.

(a) a
(b) an
(c) the
(d) (none)

English Grammar Tests, Elementary Level

ESL/EFL Test #222 "Articles (3)", answer 7

I have to buy a new lamp for the bedroom. I will go to the store on Saturday to find one that matches the other colors in the room.

Correct answer: (a) a

Your answer was: incorrect
I have to buy the new lamp for the bedroom. I will go to the store on Saturday to find one that matches the other colors in the room.
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The new lamp has been decided with the colours. So, isn't there a previous idea which to buy...!!! In that case can't we use 'the' there?

I feel it like this, "I have to buy the new lamp for the bedroom which matches the other colours in the room"

Thanks.

[Mister Micawber]

Actually, the colours have nothing to do with it; that remark could come with the decision. Just saying 'I have to buy the new lamp' indicates that it has been spoke of before. But it hasn't been spoken of before in this text, so 'a'.
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Native English teacher at Mister Micawber's

[Suneethaago]

Yes. I feel 'a' is correct.

Thank you for your explanation Mister Micawber.