Final answer:
The cost of electricity does indeed vary directly with the number of kilowatt-hour consumption, as they are related through a constant cost per kWh set by the utility company.
Step-by-step explanation:
The statement that the cost of electricity (c) varies directly as the number of kilowatt-hour consumption (n) is true. In mathematics, when we say that one quantity varies directly with another, it means that there is a constant ratio between them. For electricity, this constant is the cost per kilowatt-hour set by the electric utility. This relationship between cost and consumption can be expressed by the formula c = k * n, where k is the cost per kilowatt-hour and n is the number of kilowatt-hours used. The energy used is measured in kilowatt-hours (kWh), which is the product of power in kilowatts (P) and time in hours (t), following the equation E = Pt.