Answer:

Explanation:
Given:
A solution contains one or more of the following ions such as Ag,
and 
Here the Lithium bromide is added to the solution and no precipitate forms
Solution:
Since with LiBr no precipitation takes place therefore Ag+ is absent
Here on adding
to it precipitation takes place.
Precipitate is as follows,

Thus,
is present
When
is added again precipitation takes place.
Therefore the reaction is as follows,

Therefore,
are present in the solution
Decreasing the activation energy needed for the reaction.
The best and most correct answer among the choices provided by your question is the fourth choice or letter D.
Carbon is responsible for the variety of carbon-based molecules on Earth because it is organic in nature.
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Answer:
A
Explanation:
Since water is polar, we need for there to be a polar solute. This eliminates options B and D.
Now to choose between A and C, we can look at the compounds mentioned.
For A, NH3 is mentioned, which is a polar substance.
However, for C, H2 is mentioned, which is a nonpolar molecule, contrary to what is stated by the option.
Therefore, A is the answer.