Select the two answer choices that complete the sentence with the same meaning: Researchers comparing subjects' ability to interpret facial expressions found that some expressions were difficult because they were so _____ to the person making the expression.

This choice implies that the expressions are routine or typical for the person making them. While habitual expressions may be familiar to the individual, they do not inherently suggest difficulty in interpretation for others, as they can often be understood within the context of regular behavior.

Similar to "habitual," this option indicates that the expressions are traditional or expected for the individual. Customary expressions should generally be easier to interpret rather than difficult, as they align with common social practices or norms.

While "opaque" suggests a lack of clarity or transparency, it does not specifically imply that the expressions are uniquely tied to the individual. This choice could denote difficulty in understanding, but it lacks the personal nuance of "idiosyncratic," which more accurately highlights the individual-specific nature of the expressions.

This choice suggests that the expressions do not cause offense and are neutral in nature. However, this quality does not relate to the difficulty in interpreting facial expressions, as being inoffensive does not inherently affect clarity or understanding.