We know from this passage that out at sea, when waves going in opposite directions meet, their force ______.

A．call for an ambulance
B．prevent bleeding
C．know what to do
D．act quickly

A．One should not apply first aid to the injured without knowing the ease of the accident.
B．It is easy to carry out first aid if one has the necessary facilities.
C．First aid is an important part of the whole treatment.
D．First aid may not be effective before a doctor or expert comes.

A．be self-controlled
B．have others to assist him
C．have confidence in the sick person
D．get rid of the by-standers