It's a good thing for a／an____ to explain this difficult subject

A、the same

B、same

C、similar

D、alike

A、bad

B、good

C、weak

D、strong

A、continuous

B、continual

C、constant

D、eternal

A、of

B、with

C、for

D、on

A、runs for

B、runs against

C、runs over

D、runs into

A、would take

B、had taken

C、will have taken

D、would have taken

A、unless

B、so that

C、if

D、lest

A、set out

B、set off

C、set down

D、set to

A、having seeing

B、saw

C、to have seen

D、to see

A、turned away

B、turned back

C、turned down

D、turned over