Chemistry
In a reaction of the type A+B ? C+D one could ensure it to be a first order reaction by :

Increasing the temperature
Adding a catalyst
Increasing the concentration of a reactant
Increasing the concentration of a product

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Chemistry
The mass number of a nucleus is

a fraction
always less than its atomic number
the sum of the number of protons and neutrons present in the nucleus
always more than the atomic weight

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