2. Nosotras
Nosotras is used because the verb ends in -mos.
If it were piensas then tú is used.
If it were pienso then yo is used.
If it were piensan then ellas is used.
Enfermo should be the answer.
Explanation:
The force of attraction between all masses in the universe; especially the attraction of the earth's mass for bodies near its surface.
The SI unit of gravitation is newtons