I don't know what the options were but a material that is very likely going to be challenging to recognize under a microscope as a mixture is a homogeneous mixture. A homogenous mixture is uniform and thus hard to recognize as a mixture. An example is water!
...because this notation increases the convenience in using the numbers
Answer:
Calcium is more reactive.
Explanation:
It is higher in the reactivity series, and contains more energy levels.
