Being in zero gravity causes the (expand) of a person's spinal discs.

Being in zero gravity causes the expansion of a person's spinal discs.

Explanation:

In the given blank, the word to be placed will be the correct form of the word in parenthesis. This means that the given word "expand" will be changed according to the form of the sentence.

The blank in the sentence will be filled with the word "expansion". Now, the word "expansion" is a noun that means an increase in size, to become bigger. And in that context, the final sentence will become

Being in zero gravity causes the expansion of a person's spinal discs.

they fear for their lives

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