The right to be

[vendencially]

Hi, I've been discussing this question with my friend and still doesn't understand the right answer.
We got a task to change the active sentence and turn it into passive.
Here is the active sentence

A: She developed time management tools for staff.

My answer:
P: Time management tools for staff was developed by her

But it is incorrect based on my teacher's answer. She said the right answer is
P: Time management tools were developed by her.

I've asked my senior and she said that 'was' is the correct term because "time management tools for staff" is considered as one object, it's like making one journal even though it has several pages.

MY QUESTION IS.. which one is correct? WAS or WERE and why is that?
Does the clause 'time management tools for staff' is considered as plural or singular?

[Josef Essberger]

Your teacher is right.

P: Time management tools [for staff] were developed by her.

They are tools, it doesn't matter if they are for an army or for my grandmother. They remain plural.

PS: "Time management tools for staff" is not a clause. It has no verb. It is at most a phrase.

[vendencially]

Oh i see.. so it doesnt matter what follows after the object.. Thank you so much!!!!