A.) half - life of a radioactive substance is defined as the time during which concentration of the substance becomes half the initial value.
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Your answer should be:
N2 + O2 → 2NO
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<span>C) <u>Colloids</u></span><span>
Colloids have small non-dissolved particles that flow around in the mixture. These particles do not settle over time. When a light is shined on colloids the scattering characteristic of the Tyndall effect are visable.</span>
Carbon is the one and only element to make a molecule organic.
Answer : The correct option is, (B)
Explanation :
Balance equation : Balanced equation are those equation in which the number of atoms of each element and the total charge must be same on products side and reactants side.
In the given options, only option (B) equation has correct formula and also a balanced equation.

while the other options has incorrect formulas and also an unbalanced equations.
The correct equation for option A is, 
The correct equation for option C is, 
The correct equation for option D is, 
Hence, the correct answer is, (B) 