Why is the Phalgu River near Gaya often referred to as an 'underground' or 'invisible' river (Antarsalila) during the dry season?

Options:

• A: It flows through deep limestone caves beneath the Vindhyan sedimentary rocks.
• B: It is entirely diverted into artificial underground canals for irrigation before reaching Gaya.
• C: It merges with the Ganga through an underground tectonic fault line near Patna.
• D: Its water flows beneath a thick layer of sand on the riverbed, leaving the surface dry while water remains accessible just below.

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