the cell will expand as water moves into it.
Answer:
The mass was there all along, it was just in the air. The weight of the oxygen from the air is not weighed in the beginning, only at the end as part of the product, making it seem like there is a total mass change.
I think the answer is A but i'm not sure
Endothermic or C, if you would like a reason why, do not hestitate to ask