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What will be the number of optically active and inactive isomers 1,2,4,5-tetramethylspiro[2.2]pentane?

1,2,4,5-tetramethylspiro[2.2]pentane

The question asked for the difference between number of optically active and optically inactive isomers. When I drew them, I could find 6 optically active, and none optically inactive isomers. So, according to me answer should have been $6-0=6$. But the given answer is 5. Apparently, one optically inactive isomer is possible, but I don't see how.

I am assuming the answer given to be correct. Though, there is very slightest possibility of it being incorrect too, but I don't have any source to confirm, thefore asking on Chemistry SE. :)

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    $\begingroup$ There is one center of symmetry, giving rise to one meso compound which is optically inactive. $\endgroup$
    – J_B892
    Apr 26 '18 at 4:49