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MJD I too looked things up and found the following web reference....
Vegetables High In Water
Given that a leafy vegetable like Lettuce can have up to 96% water, the thing that it does have when compared to say, Celery, Radishes, Cucumber and Zuchini is compactness. Lettuce is leafy with lots of air space between the leafs. Therefore, even though it can contain nearly all water, it won't fit in volume in that sealed box.
Celery, Radishes, Cucumber and Zuchini are all fairly compact vegetables by contrast, meaning that choosing them for the contents of that sealed box, means that you'll have more water than if you chose Lettuce or Bok Choy.
My choice would have been Radishes and Cucumber, each at 95+% water, similar to Lettuce and Bok Choy, but more solid and compact.