According to the fda, products labeled gluten-free must contain less than of gluten.

According to the FDA, food products labeled "gluten-free" or "no gluten" must contain at least 20 parts per million in order to be in an acceptable range. Especially since people with celiac disease, when the small intestine is sensitive to gluten, can only tolerate a tiny amount.

The right answer is period (periodic motion).

A phenomenon is periodic when it reproduces, identical to itself, at the end of a certain time called period.

A periodic phenomenon is a phenomenon that is reproduced, identical to itself, at equal time intervals. An example of periodic movement in biology in the heart rhythm: even if a physical activity can modify their rhythm, the heart beats represent a good example of periodic phenomenon.