Reasonable definitions of the decision variables are:
a.
x₁₁ household without children under 16 and without premium subscription
x₁₂ household without children under 16 but with premium subscription
x₂₁ household with children under 16 but without premium subscription
x₂₂ household with children under 16 and with premium subscription
b.
x₁₁ number of interviewed households without children under 16 and without premium subscription
x₁₂ number of interviewed households without children under 16 but with premium subscription
x₂₁ number of interviewed households with children under 16 but without premium subscription
x₂₂ number of interviewed households with children under 16 and with premium subscription
c.
x₁₁ cost of interviewing a household without children under 16 and without premium subscription
x₁₂ cost of interviewing a household without children under 16 but with premium subscription
x₂₁ cost of interviewing a household with children under 16 but without premium subscription
x₂₂ cost of interviewing a household with children under 16 and with premium subscription
d. All of the above are reasonable definitions.
e. None of the above.