One intensity does not identify which wave leads

Each of two equal coherent beams separately gives 6 W/m². Together they give 18 W/m². A phase lag is known to lie between 0° and 180°.

What is that phase lag?

Organize the givens & assumptions

I₁ = I₂ = 6 W/m²
Combined intensity = 18 W/m²
0° ≤ δ ≤ 180°

Same-frequency coherent beams, parallel polarization, same observation medium and unchanged individual detector intensities, unless the prompt explicitly averages phase.

A $90^\circ$
B $120^\circ$
C $60^\circ$
D $30^\circ$

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