HELP (whats the imperfect tense for coleccionar and compartir here?)

Es importante que nosotros en el medio ambiente. a. piense b. piensen c. pensemos d. pienses

Which part of this sentence is not correct? esta muchacha tan bonito es alta. a. muchacha b. bonito c. esta d. alta

When could you attach the direct object pronoun to an infinitive verb? a: when an infinitive verb follows a conjugated verb. b: when an infinitive verb ends in a question. c: when a stem-changing verb ends in a question. d: when a stem-changing verb follows a conjugated verb.