Answer:
The St. Louis Cardinals have a rich history of success, although their offense this year was awful.
Explanation:
The two sentences are expressing contrasting ideas. The first sentence offers a type of compliment to the St. Louis Cardinals. If they have a rich history of success, however, we would not expect the criticism that comes in the second sentence. We are surprised to hear that their offense was awful this year.
<u>To help express such contrast and, at the same time, make the second sentence a subordinate clause, we should choose an appropriate subordinating conjunction. The conjunctions that can help us do that are "though", "even though", and "although", among others. Therefore:</u>
The St. Louis Cardinals have a rich history of success, although their offense this year was awful.
NOTE: We may be tempted to use "but" instead of "although". We need to keep in mind that "but" is a coordinating conjunction, not a subordinating one. That is why it cannot be used here.
