Complex compounds are broken down to simpler substances in catabolic reactions.
These kinds of reactions often occur in biological systems. In living organisms, complex compounds like lipids, proteins and complex sugar like cellulose are broken down into simpler forms. Products of these reactions are simple sugars, amino acids etc. but a certain amount of energy is also produced and stored in energy molecules for future use.
Answer:
A. Compounds
Explanation:
Firstly, don't let the word "pure" confuse you; this is pretty much irrelevant to the question.
The key to answering this question lies in "substance" and "more than one type of element chemically bonded."
Something you ought to memorize is that a substance is either an element or compound. Therefore, you can eliminate choices B./C.
Next, use the part of the definition that says "more than one type of element chemically bonded" to conclude that it's a compound. Not only is this the definition of a compound, but the fact that is says <em>more than one type of element</em> should automatically tell you that it is different from just a regular element (Choice D.).
Answer:- 3. 
and 
Explanations:- An empirical formula is the simplest whole number ratio of atoms of each element present in the molecule/compound.
For example, the molecular formula of benzene is 
. The ratio of C to H in it is 6:6 that could be simplified to 1:1. So, an empirical formula of benzene is CH.
In the first pair, the ratio of C to H in first molecule is 2:4 that could be simplified to 1:2 and the empirical formula is 
. In second molecule the ratio of C to H is 6:6 and it could be simplified to 1:1. and the empirical formula is CH. Empirical formulas are different for both the molecules of first pair and so it is not the right choice.
In second pair, C to H ratio in first molecule is 1:2, so the empirical formula is . The C to H ratio for second molecule is 1:4, so the empirical formula is 
. Here also, the empirical formulas are not same and hence it is also not the right choice.
In third pair, C to H ratio in first molecule is 1:3, so the empirical formula is . In second molecule the C to H ratio is 2:6 and it is simplified to 1:3. So, the empirical formula for this one is also . Hence. this is the correct choice.
In fourth pair, first molecule empirical formula is CH. Second molecule has 2:4 that is 1:2 mole ratio of C to H and so its empirical formula is . As the empirical formulas are different, it is not the right choice.
So, the only and only correct pair is the third one. 3. and
