D, as he is testing which type of fertiliser is the best, and hence it should be the only changed variable. Everything else should remain the same.
Electric current is the flow of charge due to the potential difference between two terminals per unit time. It is denoted by I and its unit is amp. It can be mathematically expressed as I=Q/t.
If this isn’t what your looking for I need the options they gave you
Gravitation - greater objects in space attract the lesser objects
<span>Force of steam engine - caused by atmospheric pressure when the condensed vapor reduces its volume under the piston and vacuum is created </span>
<span>Inertia - when rapid object is stopped
Hope it helps. </span>