Question #N643

The team’s efforts to develop a new type of battery that would be both environmentally friendly and cost-effective have been a long process. The team has been working for years, and they have made significant progress in their research. The team has encountered many challenges along the way, but they have learned from these challenges and have continued to strive towards their goal.

Which choice best combines the sentences at the underlined portion to maintain the formal style of the passage?
A. The team has been working for years, and they have made significant progress in their research, but they have encountered many challenges along the way.
B. The team has been working for years, and they have made significant progress in their research, but they have encountered many challenges along the way, and they have learned from these challenges.
C. The team has been working for years, and they have made significant progress in their research; however, they have encountered many challenges along the way.
D. The team has been working for years, and they have made significant progress in their research; although they have encountered many challenges along the way.