People did not begin to use unmanned vehicles in undersea exploration until recently because of______.

A．New techniques are continually developed and new machines invented for undersea exploration.
B．If not well built, a submersible is most likely to get crushed in deep water.
C．A submersible, when manned, looks very like a spaceship.
D．In certain respects undersea exploration is often faced with the same problem as the space exploration.