The Kilian-Fischer synthesis lengthens an aldose carbon chain by adding one carbon atom to the aldehyde end of the aldose. The result of this process is a chain lengthened sugar with a new carbon atom at the C1 and the former aldehyde group now at `C_(2)`. This synthesis is useful both for determining the structure of existing sugars and for synthesizing new sugars.
The most common method used to shorten sugar chains is the Ruff degradation. The Ruff degradation is used mainly for structure determination and synthesis of new sugars.
Ruff degradation of D-arabinose gives D-erythrose. The Kilian-Fischer synthesis converts D-erythrose to a mixture of D-arabinose and D-ribose. Draw out these reactions, and give the structure of D-ribose.

Explanation

To solve the question regarding the Kilian-Fischer synthesis and Ruff degradation, we will break it down into steps and illustrate the reactions involved.

### Step 1: Structure of D-Arabinose
D-Arabinose is a five-carbon aldose sugar. Its structure can be represented in Fischer projection as follows:

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CHO
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