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The poem is
ἡ γῆ μέλαινα πίνει,
πίνει δένδρεα δ᾿ αὐτήν.
Frequently a neuter plural takes 3rd person singular. I am sure this will be in your textbook which I don’t have to hand but there was another thread recently where this came up.
See here http://discourse.textkit.com/t/mastronarde-unit-22-sentences-for-reading-number-2/16706/1