code: def test_add_trend_c():
      x = np.zeros((10, 1))
      y = add_trend(x, trend="c")
      assert np.all(y[:, 1] == 1.0)
  
imports: import numpy
  from numpy.random import RandomState
  from numpy.testing import assert_array_almost_equal, assert_array_equal, assert_equal
  import pandas
  from pandas.testing import assert_frame_equal
  import pytest
  from arch.utility.timeseries import ColumnNameConflict, add_trend
label_desc: Use image node_id 7 for calling a global function with example usage: test_add_trend_c() without return types