Calculate total energy consumption in kWh and multiply by cost per unit

Steps:

• Total days: 3 weeks × 7 days/week = 21 days.
• Total hours: 21 days × 5 hours/day = 105 hours.
• Energy used: 100 W = 0.1 kW; 0.1 kW × 105 hours = 10.5 kWh.
• Total cost: 10.5 kWh × $0.24/kWh =$2.52.

Why C is correct:

• Energy cost formula: total kWh = power (kW) × time (h), then cost = kWh × rate per kWh, yielding exactly $2.52.

Why the others are wrong:

• A: Underestimates time, perhaps only 5 hours total (0.1 kW × 5 h × $0.24 =$0.12, but doubled incorrectly).
• B: Uses 1 week only (0.1 kW × 35 h × $0.24 =$0.84).
• D: Assumes 20 days (0.1 kW × 100 h × $0.24 =$2.40, close but wrong).

Final answer: C