Which of the following words is an EXCEPTION to a common spelling rule?

Pastime is spelled correctly, but it is an exception to the following spelling rule: Don’t Omit Letters for Compound Words. The other words conform to the following rules:
- Bicycle: Adding a prefix to a word does NOT change the spelling of a word.
- Beautiful: If a word ends in y preceded by a consonant, change the y to i, then add the suffix.
- Stopping: Double a final single consonant before adding a suffix when the word ends with a single vowel followed by a single consonant AND the consonant ends a stressed syllable or a one-syllable word.