Answer:
[tex]12unit^2[/tex]
Explanation:
Assuming that the screen covers the entire area of the smart phone.
[tex]3 * 4 = 12 unit^2[/tex]
Most likely this wouldn't be the case, so the screen size would be at most [tex]12unit^2[/tex]
[tex]0unit^2<size < 12unit^2[/tex]
And would most likely fall under this range of answers.
