A until
B unless
C as soon as
D while
Explanation: \'As soon as\' indicates immediately after something happens.
Explanation: \'Are\' is the correct auxiliary for present continuous with \'they.\'
Explanation: \'On\' is used when something rests on a surface.
A then
B than
C as
D like
Explanation: \'Than\' is used for comparisons between two things.
A will
B would
C shall
D can
Explanation: In first conditional sentences, \'will\' is used in the main clause.
A hardly
B hard
C harder
D hardest
Explanation: \'Hard\' means with great effort; \'hardly\' means almost not at all.
A has...eaten
B have...eaten
C had...eat
D has...eating
Explanation: \'Has eaten\' is present perfect tense with third person singular.
A have been
B has been
C had been
D are been
Explanation: \'Have been waiting\' is present perfect continuous for \'we.\'
A has gone
B went
C goes
D has been going
Explanation: \'Went\' is simple past, used with a finished time expression \'last summer.\'
A saw
B see
C have seen
D am seeing
Explanation: \'Have seen\' is present perfect, used for past experience with relevance to now.
A has...left
B had...left
C was...leaving
D did...leave
Explanation: \'Had left\' is past perfect, used for an action completed before another past action.
A a
B an
C the
D no article
Explanation: \'A\' is used before consonant sounds; \'European\' starts with a \'y\' sound.
A a
B an
C the
D no article
Explanation: \'The\' is used for specific, known items (the salt on the table).
A A
B An
C The
D No article
Explanation: Abstract nouns used in a general sense do not require an article.
A for
B since
C from
D during
Explanation: \'For\' is used with a duration of time; \'since\' is used with a point in time.
Explanation: \'At\' is used with specific times.
Explanation: \'Fond of\' is the correct prepositional phrase.
Explanation: \'But\' is used to connect two contrasting ideas.
Explanation: \'And\' connects the condition to the positive result.
Explanation: \'Or\' is used in \'either...or\' constructions to present alternatives.