The authors use the word "backbone" in lines 3 and 39 to indicate that

Only very long chains of DNA can be taken from an organisms with a spinal column

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The main structure of a chain in a DNA molecule is composed of repeating units

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A chain in a DNA molecule consists entirely of phosphate groups or of sugars

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Nitrogenous bases form the main structural unit of DNA

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Solution

The correct option is B The main structure of a chain in a DNA molecule is composed of repeating units
The author of the passage explains the structure of DNA in detail. The term 'backbone' used in line 3 and 39 explains that the molecule is a very long chain which consists of a regular alternation of sugar and phosphate groups.
Thus, option B is the correct answer.

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