I didn't understand why the verb to be is in this setence!

[Hanfy]

English Grammar Tests, Elementary Level

ESL/EFL Test #127 "Real Life: Occupations (2)", question 1

My friend is a school ......... driver. She picks up children on her daily route. She drives them to school safely in the morning and then she picks them up again in the afternoon once they've finished school.

(a) truck
(b) bus
(c) taxi
(d) delivery

English Grammar Tests, Elementary Level

ESL/EFL Test #127 "Real Life: Occupations (2)", answer 1

My friend is a school bus driver. She picks up children on her daily route. She drives them to school safely in the morning and then she picks them up again in the afternoon once they've finished school.

Correct answer: (b) bus
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I didn't understand why the verb to be is in this setence "My friend is a school bus driver".

Hanf

[Kitosdad]

Having only "am", "is", and "are" to choose from, then "is" is the the most logical and grammatically correct verb to use in this sentence.
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Englishman living in Germany.