Answer:
A. The conditions are:
I. Reactant particles must collide with the right orientation.
II. There must be effective collisions.
III. The reactant particles must possess enough energy to break old bonds so that new bonds can be formed.
B. The activated complex occurs where the maximum energy of the reaction is attained along the reaction pathway, that is, at the peak of the activation energy.
Explanation:
Molar mass of HBr = 81 g/mol
Molar mass of nitrogen dioxide gas = 46 g/mol
Molar mass of ethane = 30 g/mol
Graham's Law states that the rate of effusion or diffusion of gas is inversely proportional to the square root of the molar mass of the gas. The equation given by this law follows the equation:

So, the gas with least molar mass will effuse out fastest from the container and that is ethane gas.
The formula for average kinetic energy is:

where,
k = Boltzmann’s constant = 
T = temperature = 273.15 K ( at STP)
As we can see from the formula that kinetic energy depends upon only temperature of the gas molecule.
So, from this we can say that all the gas molecules have the same average kinetic energy at this temperature.
C. The mass number is the amount of Protons and electrons there are combined, so 200 - 80 is 120.
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Small bodies from which the planets are formed is called : Planetesimal.
Planetesimals that have survived to the current day are valuable to scientists since they contain information about the Solar System's formation.