Final answer:
Both sentences provided are complete with clear subjects and predicates. Effective sentences require a balance between conciseness and clarity, with sentence variety enhancing readability and flow.
Step-by-step explanation:
Both of the given examples are complete sentences. The first sentence 'Every year, Billy competes to build the world's tallest gingerbread house.' clearly has a subject 'Billy' and a predicate 'competes to build the world's tallest gingerbread house', making it a complete sentence. Similarly, the second sentence 'Eating the walls of his huge creation is the best part.' also contains a subject 'Eating the walls of his huge creation' and a predicate 'is the best part', qualifying it as a complete sentence as well.
When revising for more effective sentences, remember that sentence variety and clarity are key. Combining sentences can prevent repetition and provide a more fluent reading experience. For achieving brevity without sacrificing effectiveness, it's imperative to balance concise language while retaining the intended impact of the sentence, which may depend on the context and purpose of the writing.