What best describes a pull-up or pull-down resistor?

  1. A resistor in a keying circuit used to reduce key clicks
  2. A resistor connected to the positive or negative supply line used to establish a voltage when an input or output is an open circuit
  3. A resistor that ensures that an oscillator frequency does not drift
  4. A resistor connected to an op-amp output that prevents signals from exceeding the power supply voltage