Water and dimond are the 2 pure substances
Dmitri Mendeleev is the scientist who first developed the periodic table.
He is a Russian chemist and inventor who formulated the periodic law and created the periodic table of elements. He corrected some of the properties of the elements. Now, we can easily locate the elements we wanted to identify. With the help of Mendeleev, studying chemistry is not that difficult because he tried to simplify some of the concept that was complex before.
Answer:
B. 
Explanation:
The unit for rate is M/s while the unit for each molecule should be M. You can find the unit for k by putting the units for rate and the molecules into the equation
rate= k{X][Y]
M/s= k *
* 
k= (M/s) / (
)
k= 
You can also use this predetermined formula to solve this problem faster: k= 
Where n is the number of molecule. There are 3 molecule(2X and 1Y) so n=3, so
k= 
k=
=
= 