Step 1: Understanding compounds.
Compounds are words that are made by combining two or more smaller words to form a new meaning. For example, "blackboard" is a combination of "black" and "board," and "toy factory" is a combination of "toy" and "factory."
Step 2: Analyzing the options.
- (A) Incorrect, as "repeat" and "backward" are not compound words, they are individual words.
- (B) Correct, as "blackboard" (black + board) and "toy factory" (toy + factory) are both compound words.
- (C) Correct, as "sky-high" (sky + high) and "underestimate" (under + estimate) are both compound words.
- (D) Incorrect, "bystander" is a compound word, but "deform" is not.
Step 3: Conclusion.
Thus, the correct answers are (B) blackboard and toy factory, (C) sky-high and underestimate.