A $1$ kg copper block $(c=390\frac{J}{k}\frac{g}{K})$ is heated to $100C,$ then dunked in an insulated tub containing $1$ kg of water $(c=4186\frac{J}{k}\frac{g}{K})$ at $0C.$ What is the final equilibrium temperature?

$100C$

$50C$

Between $50C$ and $100C$

$0C$

Between $0C$ and $50C$