2. ¿Usaste la computadora?
a. Si los usé.
b. Si las usé.
c. Si lo usé.
d. Si la usé.

Q: (Did you use the computer?)

D) Yes I used it.

Ser is used to describe the permanent characteristics of a person place or thing. You use it for descriptions, occupations, characteristics, time and origin.

Estar is used to talk about HOW things are. You use it for locations, emotions and TEMPORARY actions.

SUMMARY:

SER:
Date
Occupation
Characteristics
Time
Origin
Relation

ESTAR:
Position
Location
Action
Condition
Emotion