Reactant are those that combine or reacts to give products !!
so in combustion of ethane ; ethane and o2 are reactants so ... your answer is B !!
% m/v of the solution = 0.2%m/v
<h3>Further explanation</h3>

or
% m/v = g solute / 100 ml solution
Concentration : 2 g sodium borate / L water
For 10 L⇒20 g Sodium borate

It should be D, cells are parts, tissue is part and organ is part of a system.
im missing something though
<span>Radioactive isotope, also called radioisotope, radionuclide, or radioactive
nuclide, any of several species of the same chemical element with
different masses whose nuclei are unstable and dissipate excess energy
by spontaneously emitting radiation in the form of alpha, beta, and
gamma rays.</span>