To work like a horse

Please read the following letter and try to guess the meaning of the idiom to work like a horse.
Dear Sheila,

How are things with you, my dear sister? I hope all is well.
I am still trying to adapt to New York’s life style. I can’t believe I’ve been here for almost six months now! I have been thinking about moving back to Los Angeles. At least there I have you, Mom and Dad, my friends, and I would be able to get my old job back.
My work schedule is crazy! I work from 9:00 a.m. to 9:00 p.m. almost everyday. I know I am lucky to have such a well-paying job having just graduated from college, but I work like a horse and then go home and sleep. I don’t have the time or energy to enjoy all the money I am making… I am sorry to be so negative, but I am really upset and have no idea if I’ll be able to do this anymore.
I will write again after I make a decision.
I can’t write anymore now. I have a meeting starting in 3 minutes.

I love you,

Timothy.

What do you think to work like a horse means?

a. to work at the zoo
b. to work too much
c. to work just a little
d. to work dressed as a horse

Meaning:

To work like a horse means to work too much.

This idiom was taken from the LSI level 4 L/S book Speaking Transitions.