A synchronous TDM is used to multiplex two $50$ kbps$,$ two $30$ kbps and one $20$ kbps signals, each with a data unit of $1$ bit. Which of these possible frame rates and number of bits per frame would be the most efficient $($lowest bandwidth$)$ for the multiplexer?

options:

$50$ kfps and $5$ bits$/$frame

$30$ kfps and $7$ bits$/$frame

$10$ kfps and $18$ bits$/$frame

$10$ kfps and $36$ bits$/$frame