Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. A. Planetesimals may remain in star systems when hydrogen and helium combine with less common heavier elements. B. Planetesimals are composed of heavy elements because hydrogen and helium stay in the form of gases. C. Planetesimals are small, solid objects that condense within a forming star system and may become planets. D. When planetesimals accumulate to form planets, they inevitably contain gaseous as well as heavy elements.