Human breast milk contains large quantities of secretory Ig A (sIgA).
I think that the best word to fill into the gap is "solid".
Solid comes in opposition to other fundamental states of matter: liquid, gas, plasma (the last one can be characterized by high electric conductance).
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The cell requires a barrier between itself and its environment.
Every time when there is a full moon, Mrs. Cook insists that students in her classes display strange behavior and the best way for her to prove the validity of her theory is to research data to find concrete evidence of a link between the moon cycle and human behavior. Hope this helps. The answer is B