This might be a confusing question especially from someone who is not from the USA. You need to be familiar with your constitution because this will make answering this question easier. The one who will be in charge is letter B. In countries that follow a democratic government, the Senate president and the vice president is different.
This means that if the vice president is unable to do his/her task, the Senate president will take over The Senate president will be placed in position by the other senators. In the US, the vice president is also the Senate president. This will make the president pro tempore the best person to take over the needed duties.
The president pro tempore-the vice-president of the u.s. is also president of the senate. the president pro tempore (literally, president for a time) is chosen by the senate and presides when the vice president is absent (article i, section iii). the president pro tempore is also third in the line of succession to the presidency of the u.s. this is not to be confused with the position of senate majority leader, which is the political head of the senate. he or she determines legislative strategy and is often the most powerful officer in the senate.