Square root radical equation problem.?

Solve √(5+y^2) +√(5 - y^2) = 4 where y∈ R .

Choices.

A. y= ± 1
B. y = - 2 only
C. y = 2 only
D. y = ± 2

my answer from my solution is y = ± 2, but how can I determine its y∈ R?
4 answers 4