Unless until the newly thin over-weight people metabolism relates to their new size they will regain weight.
B says the varying metabolism rate of NOT over weight persons. So they can gain or lose weight easier than the over weight people. But how does this premise will act as a base for the above mentioned Conclusion. We have mentioned nothing about the over-weight people metabolism in the option. In the passage also it is mentioned that over-weight people burn less calories than the normal people. So we cannot be concerned about this option.
On the other hand A says Newly became thin people metabolism is still that of an over wight people. So the metabolism burns very few calories. So if the intake is very few calories then metabolism will burn that few calories and there will be very less excess of it to help regain weight.
