4.1.3 However, [when we look at（观察） the 100 billion stars (恒星）in our galaxy(银河)(the Milky Way),and 100billion galaxies（星系）in the observable Universe（可见宇宙）], it seems inconceivable(难以置信的)that at least one of these planets does not have a life form on it.The trunk in the above sentence is__________.

A. SVP-It（形式主语） seems inconceivable.
B. SVO-We look at the 100 billion stars and 100billion galaxies.
C. SVO- It（形式主语） seems inconceivable.
D. SVO-It least one of these planets does not have a life form.