although ...but

Sentence A: Remember the pill is highly effective, but pregnancy can occur if the pill is taken at irregular times, if intercurrent illnesses such as fever and gastric upsets develop, or if you are taking some other drugs.



Sentence B: Remember although the pill is highly effective, but pregnancy can occur if the pill is taken at irregular times, if intercurrent illnesses such as fever and gastric upsets develop, or if you are taking some other drugs.



Sentence C: Remember although the pill is highly effective, pregnancy can occur if the pill is taken at irregular times, if intercurrent illnesses such as fever and gastric upsets develop, or if you are taking some other drugs.



My questions:

All of the three sentences acceptable? If yes, which one is the best.





Thanks in advance!