<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_reference_band(self, data):
 """ Extract the NDVI band from time-series :param data: 4D array from which to compute the NDVI reference band :type data: numpy array :return: 3D array containing the NDVI reference band """

        nir = data[..., self.nir_idx].astype("float32")
        red = data[..., self.red_idx].astype("float32")
        return (nir - red) / (nir + red)