TOEFL Reading: ETS-TOEFL阅读机经 - 9V6XYU8KV335569XL$

According to paragraph 4, what is true about river deltas A. They alternate between periods of spreading out and periods of getting smaller. B. They have both active channels and channels that have been blocked by sediment. C. They are typically formed by rivers that are braided before they reach the delta area. D. They are much larger when formed in a still lake than when formed in the ocean.