Scientists think that algae first evolved approximately 2.7 billion years ago, and soon after this oxygen began to exist in the atmosphere. Photosynthesis by primitive plants and algae released oxygen, which gradually built up in the atmosphere.
Explanation:
Answer:
1. Caminen al parque Uds.
2. No estén tristes Uds.
3. Viaje Ud. a Puerto Rico.
4. Escuchen el radio Uds.
5. Visiten a sus abuelos Uds.
6. Sirvan Uds. las papas fritas.
7. Vaya Ud. a la playa.
8. Bailen la salsa Uds.
9. Dé Ud. el regalo.
10. Coman Uds. la cena.
11. Escriba Ud. la carta.
12. Prepare Ud. la cena.
13. Tengan cuidado Uds.
14. Sea feliz Ud.
15. Hagan la cama Uds.
16. Esté contento Ud.
17. Den la mano Uds.
18. Vayan Uds. al supermercado
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