Biology question cell membranes

Hi, please could I have help on this question? I don’t understand why the answer is C?
Here is the question: https://app.gemoo.com/share/image-annotation/630963184736882688?codeId=vzROpZQlrkrOr&origin=imageurlgenerator
Thanks!

Original post by anonymous294

Alveoli epithelial cell - twice, enter + leave
Blood vessel endothelial cell - twice
Erythrocyte - to enter. Once

(edited 1 month ago)

Original post by BankaiGintoki

Alveoli epithelial cell - twice, enter + leave
Blood vessel endothelial cell - twice
Erythrocyte - to enter. Once

But why is it entering and leaving, isnt it just travelling to haemoglobin?

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