HELP, 50 POINTS! Which example needs a transition word or phrase between sentences to connect the ideas?

a.)
Engaging your employees in a day of service is a good way to make a difference in your community. The costs associated with shutting down your business for a full day may be significant.

b.)
Several significant hurricanes have hit the region in recent years. The damage done by them has caused insurance rates to rise.

c.)
Many students don't clearly understand what constitutes plagiarism. Therefore, plagiarism is often unintentional.

d.)
Real estate sales have moved in tandem with the stock market for the last 15 years. When the stock market has expanded, real estate sales have increased.