单选题 .  What was the origin of the oil which now drives our motorcars and aircraft? Scientists are     21    about the     22    of coal, but they do not seem so     23    when asked about oil. They think that the oil     24    the surface of the earth originated in the     25    past, and was formed     26    living things in the sea. Countless billions of     27    sea creatures and plants lived and     28    to the sea bed. They were covered with huge     29    of mud; and by     30    of chemistry, pressure and temperature were changed through long     31    into what we know as oil. For these creatures to become oil, it was necessary that they should be     32    between layers of rock for an enormous     33    of time. The     34    that oil originated in the sea is     35    by a glance at a map     36    the chief oil fields of the world; very     37    of them are far distant from the oceans of today. In some places gas and oil     38    to the surface of the sea from its bed. The rocks in which oil is found are     39    marine origin too. They are sedimentary rocks, rocks which were     40    by the action of water on the ocean.21.