In a single phase full wave regulator feeding an R-L load, the power factor angle of load is θ. The range of control of firing angle a is

• 0 ≤ a ≤ p
• θ ≤ a ≤ p
• 0 ≤ a ≤ (p - θ)
• θ ≤ a ≤ (p + θ)