PLZ HELP WILL GIVE BRAINLIEST FOR CORRECT ANSWER!!!

Read the following sentences.

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.