Couldnt 'of that' or 'because' be used here instead of 'for'?

[Mirka]

Synonyms Lesson, Advanced Level

ESL/EFL Test #110 "Synonyms for alone and lonesome", question 10

The nights were the worst part for she was haunted by ......... thoughts which were killing her.

(a) alone
(b) lonely
(c) lonesome
(d) single

Synonyms Lesson, Advanced Level

ESL/EFL Test #110 "Synonyms for alone and lonesome", answer 10

The nights were the worst part for she was haunted by lonesome thoughts which were killing her.

Correct answer: (c) lonesome

Your answer was: incorrect
The nights were the worst part for she was haunted by single thoughts which were killing her.
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I am not sure about 'for' in this sentence. Could any one explain the reason of using 'for' to me? Couldnt 'of that' or 'because' be used here instead of 'for'? Is there anything i leave out or can't see? I think it would be better if 'beccause' was used here. Am i right?

Thank you.
Mirka

[Mister Micawber]

.'
Yes, 'for' means 'because', and either can be used here; it was the writer's choice to use 'for'. The only differences in usage are that only 'because' can begin a sentence, and 'for' is considered a bit more formal.
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Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber