SC

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The existence of water on Mars has been the focus of intense investigation; << in addition to the possible holding of small quantities in subsurface deposits and deep pits >>, the planet is also expected to retain large amounts of water in its polar ice caps.

(A)
in addition to the possible holding of small quantities in subsurface deposits and deep pits


(B)
in addition to the possibility to hold small quantities in subsurface deposits and deep pits


(C)
besides the possibility that it holds small quantities in subsurface deposits or deep pits


(D)
besides possibly holding small quantities in subsurface deposits or deep pits


(E)
beside possibly holding small quantities in subsurface deposits or deep pits

[mv12]

C looks fine.

[patanjali.purpose]

The existence of water on Mars has been the focus of intense investigation; << in addition to the possible holding of small quantities in subsurface deposits and deep pits >>, the planet is also expected to retain large amounts of water in its polar ice caps.
(A)in addition to the possible holding of small quantities in subsurface deposits and deep pits
(B)in addition to the possibility to hold small quantities in subsurface deposits and deep pits
(C)besides the possibility that it holds small quantities in subsurface deposits or deep pits
(D)besides possibly holding small quantities in subsurface deposits or deep pits
(E)beside possibly holding small quantities in subsurface deposits or deep pits

A - the possible holding (wordy compared to D; do not see any grammatical error)
B - the possibility to hold (TO HOLD = suggests sth for a purpose)
C - the possibility that it holds (wordy compared to A/D; do not see any grammatical error)
D/E - possibly holding (CORRECT)

E wrong for use of BESIDE; "subsurface deposits and deep pits / subsurface deposits OR deep pits" - both ok

IMO use of ALSO ok in main clause of the 2nd part of the sentence - besides the clause is not underlines.

IMO D