Flash mobs have been happening since 2003. Lately, they have become a trend.

Which of the following correctly combines these two sentences into a complex sentence?

A.
Flash mobs have been happening since 2003, but lately, they have become a trend.
B.
Flash mobs have been happening since 2003, and lately, they have become a trend.
C.
Although flash mobs have been happening since 2003, lately, they have become a trend.
D.
Flash mobs have been happening since 2003, lately, they have become a trend.

Question

Flash mobs have been happening since 2003. Lately, they have become a trend.
 
Which of the following correctly combines these two sentences into a complex sentence?
     
A.
Flash mobs have been happening since 2003, but lately, they have become a trend.
B.
Flash mobs have been happening since 2003, and lately, they have become a trend.
C.
Although flash mobs have been happening since 2003, lately, they have become a trend.
D.
Flash mobs have been happening since 2003, lately, they have become a trend.

anonymous · Answer

If by "trend" you mean "especially popular" contrasting their limited popularity in the past, a possible answer is A since "but" creates a contrasting relationship with the present situation. The problem with A is the comma after "lately". It should either be removed as unnecessary or an additional  comma should be added after "but" to isolate "lately". THEREFORE, the best answer is C since the adverb "lately" is isolated with commas.

anonymous · Answer

I'm going to agree with @bokomaru on that. C is your best bet.