Mark the letter A, B, C, or D to indicate the word(s) OPPOSITE in mean

Mark the letter A, B, C, or D to indicate the word(s) OPPOSITE in meaning to the underlined word(s) in each of the following questions.

Animal communication is not a straightforward subject. It's complicated because animals communicate differently with each other.

• A. A. impossible to express
• B. B. difficult to understand
• C. C. easy to interpret
• D. D. interesting to study