Match the atoms which might substitute for one another. match the items in the left column to the items in the right column.

1. si a. na
2. k b. v
3. nb c. c
4. cu d. ag
5. p e. ca
6. mg f. as

If you want to answer this question, all you have to do is take a look at the periodic table. An element might substitute for another element if it is from the same family/column on the periodic table.
So, these are the atoms which might substitute for one another:
1. Si = C.C
2. K = A. Na
3. Nb = B. V
4. Cu = D. Ag
5. P = F. As
6. Mg = E. Ca

The water cycle increase the water content in the earth atmosphere