How to convince my parents to get me a ferret! Please Help ;(?

Paula Asked: How to convince my parents to get me a ferret! Please Help ;(?

I'm in 8th grade and my sister is in 10th. We both would love to have a ferret
I'm very responsible, and i actually want to become a veterinarian when i am older.
I have enough time to take care of it. Go on walks, play with it..
I always wanted to have one since i was a little girl, but I was too young then.
Now I'm older and I really want one. I had lots of different kinds of animals so i think i can handle a ferret.
But my parents wont let me, my Mom says we have too much pets. A dog, a cat (which never is inside) and 2 gerbils.
They also say 'Where is it gonna go when we go on holidays' I have enough friends that would love to take my ferret for a week or two.
My Dad also says 'They stink!'Males stink way more than a female, most ferret farms descent their ferrets before they are shipped off to the pet stores. And i would also spay/neuter my ferret.
I would clean it cage weekly from top to bottom and clean its litter every few days. I would also scoop it twice a day.
I done a lot of research and still doing it, I really want one!
Any ideas how to convince my parents?