你用的单数，看了看标准答案，也是单数，估计做整体用。ensure我开始也想用来着，但是不太确定。
synonyms ENSURE,
INSURE,
ASSURE,
SECURE mean to make a thing or person sure.
ENSURE,
INSURE and
ASSURE are interchangeable in many contexts where they indicate the making certain or inevitable of an outcome, but
ENSURE may imply a virtual guarantee <the government has
ensured the safety of the refugees>, while
INSURE sometimes stresses the taking of necessary measures beforehand <careful planning should
insure the success of the party>, and
ASSURE distinctively implies the removal of doubt and suspense from a person's mind <I
assure you that no harm will be done>.
SECURE implies action taken to guard against attack or loss <sent reinforcements to
secure their position>.
