'The fire was caused' vs 'The fire were caused'

[Milka Way]

English Grammar Tests, Elementary Level

ESL/EFL Test #95 "auxiliary verbs: was, did, have", question 8

The fire ......... caused by an electrical problem.

(a) was
(b) were
(c) did
(d) had

English Grammar Tests, Elementary Level

ESL/EFL Test #95 "auxiliary verbs: was, did, have", answer 8

The fire was caused by an electrical problem.

Correct answer: (a) was

Your answer was: incorrect
The fire were caused by an electrical problem.
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why shouldn't i use were?

Milka Way

[Mister Micawber]

.
'Fire' is a singular noun here.
.
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Native English teacher at Mister Micawber's