( ( ( The ( four planets ) ) ( ( closest ( to ( the Sun ) ) ) ( -LRB- ( ( ( ( ( ( ( ( Mercury , ) Venus ) , ) Earth ) , ) and ) Mars ) -RRB- ) ) ) ) ( ( ( ( ( have ( ( high densities ) ( because ( they ( ( are mostly ) ( composed ( of rock ) ) ) ) ) ) ) , ) and ) ( are ( called ( the ( Terrestrial Planets ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (CD four) (NNS planets)) (ADJP (ADJP (ADJP (JJS closest)) (PP (TO to) (NP (DT the) (NNP Sun)))) (PRN (-LRB- -LRB-) (NP (NP (NNP Mercury)) (, ,) (NP (NNP Venus)) (, ,) (NP (NNP Earth)) (, ,) (CC and) (NP (NNP Mars))) (-RRB- -RRB-)))) (VP (VP (VBP have) (NP (NP (JJ high) (NNS densities)) (SBAR (IN because) (S (NP (PRP they)) (VP (VBP are) (ADVP (RB mostly)) (VP (VBN composed) (PP (IN of) (NP (NN rock))))))))) (, ,) (CC and) (VP (VBP are) (VP (VBN called) (NP (DT the) (NNP Terrestrial) (NNPS Planets))))) (. .)))
The four planets closest to the Sun (Mercury, Venus, Earth, and Mars) have high densities because they are mostly composed of rock, and are called the Terrestrial Planets.
(ROOT (S (NP (NNP Venus)) (VP (VBZ has) (NP (DT the) (JJS most) (NNS volcanoes))) (. .)))