Select the two answer choices that complete the sentence: Researchers comparing test subjects' ability to interpret facial expressions found that while some expressions were easy to interpret, others were difficult because they were so _____ the person making the expression.

This choice suggests that expressions are routine for a person, which does not inherently explain why they would be difficult to interpret by others. While habitual expressions may be familiar, they can still be easily understood by observers, making this choice less relevant in the context of interpretation difficulty.

"Customary for" implies that the expressions are typical or traditional within a certain cultural or social context. However, customary expressions generally maintain a level of clarity and recognition for observers, contradicting the idea of difficulty in interpretation.

While "opaque to" suggests a lack of clarity, it is more indicative of something being unclear rather than unique. An expression might be difficult to interpret due to being opaque, but this does not directly relate to the individual nature of the expression, thus making it less appropriate in this context.

This choice implies that the expressions are non-threatening or gentle. However, inoffensive expressions do not typically correlate with difficulty in interpretation, as they can often be understood universally without ambiguity.