how many seconds does it take an object to fall 98m if it start from the rest .(using gravity 10m/s²?

Neither.

Explanation:

After 10 seconds the object will have travelled 490.5m, not 98.1. The vertical distance covered in freefall by an object in time t is given by Distance= 1/2 x 9.81 x t^2. Simply multiplying 9.81 by the time will NOT give the distance covered.

The acceleration of the object will not change, it will always be 9.81 m/s^2. Its speed will keep increasing by 9.81 m/s every second, thats what the ‘per second squared*’ means. The speed after 10 seconds will be 98.1 m/s

To summarise, after 10 seconds, the object will have covered 490.5 metres, and it will have a final speed of 98.1 m/s. It’s acceleration will remain 9.81 m/s^2 at all times

the independent variable, or the one that is intentionally manipulated is the speed.

an electric circuit