Do one's duty to/with/by smb?

[linhtho0211]

Please help me to fill in the following blanks with the correct preposition:

• I have always done my duty ___ my students.

I am extremely confused about this sentence. The key says "to", my teacher says "with" but I'd rather "by".

• The boy was called ___ by his grandfather.

I think the sentence means that his grandfather named the boy. I've looked up the dictionary for the phrasal verb of call meaning as name but found nothing.

• These are sold ___ five selling each.

I would fill with "by". But my teacher fills with "at". She says that selling is an monetary unit like $. Yet, I have never known any monetary unit called selling

[mykwyner]

* I have always done my duty ___ my students.

I am extremely confused about this sentence. The key says "to", my teacher says "with" but I'd rather "by". I like "for" here.

I am extremely confused about this sentence. The key says "to", my teacher says "with" but I'd rather "by".

* The boy was called ___ by his grandfather.
In this case, I think "called" means "summoned."

These are sold ___ five selling each.
This one doesn't make any sense to me.

[linhtho0211]

* The boy was called ___ by his grandfather.
In this case, I think "called" means "summoned."

So what preposition would you fill in this blank with?