A problem by crazy singh

A steel scale measures the length of a copper rod as $L$ cm when both are at 20 degree Celsius, the calibration temperature for the scale. If the coefficient of linear expansion for steel and copper are $a$ and $b$ respectively, what would be the scale reading ( in cm) when both are at 21 degree Celsius.

1. $L* \frac{1+b}{1+a}$

2. $L* \frac{b}{a}$

3. $L* \frac{a}{b}$

4. $L$