Radioactive material undergoes 1st order decay kinetics.
For 1st order decay, half life = 0.693/k
where k = rate constant
k = 0.693/half life = 0.693/8.02 = 0.0864 day-1
Now, for 1st order reaction,
k =

Given: t = 6.01d, initial conc. = 5mg
∴0.0864 =

∴ final conc. = 2.975 mg
Answer:
D
Explanation:
As you move down anything, towards the earth, (gravitational) potential energy decreases. This eliminates options B and C.
By the law of conservation of energy, the energy of the system stays the same. However the energy may be converted into different forms. In this case, the potential energy is converted into kinetic energy - this is what makes people move faster as they go down the slope. This makes option A wrong and option D right.
Answer : The question is to write the elements alphabetically and seperating them with commas.
So in this case for your question of Fireworks , K

,
The answer will be
K, N, O.
You have to just differentiate the elements and write them according to the alphabetical order.