Final answer:
The correct form of the verb 'ser' in this sentence would be 'Somos.' 'Ser' is used for permanent qualities or characteristics.
Explanation:
Somos would be the correct form of the verb 'ser' to use in this sentence. 'Ser' is used for essential qualities or characteristics that are considered permanent, such as emotions. In this case, 'tristes' (sad) is likely describing a more enduring state of being.
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