input: 
  import numpy as np
  
  def generate_voltage_signal(frequency: int, amplitude: float, noise_std: float, num_samples: int) -> np.ndarray:
      """ Generate a voltage signal with random noise. The signal is a sine wave
      with the given frequency and amplitude, and random Gaussian noise is added 
      to the signal.
      >>> signal = generate_voltage_signal(250, 1, 0.1, 1000)
      >>> len(signal)
      1000
      >>> np.allclose(signal.mean(), 0, atol=0.1)
      True
      """
      t = np.linspace(0, 1, num_samples)
      voltage_signal = amplitude * np.sin(2 * np.pi * frequency * t) + np.random.normal(0, noise_std, num_samples)
      return voltage_signal
  
output:     t = np.linspace(0, 1, num_samples)
      voltage_signal = amplitude * np.sin(2 * np.pi * frequency * t) + np.random.normal(0, noise_std, num_samples)
      return voltage_signal