Which shows the correct use of a semicolon to combine sentences? 
 a. In 1876, Philadelphia held a world’s fair; Edmonia Lewis sent her sculpture to be exhibited there. 
 b. The city of Pompeii was built close to a volcano; when volcanoes erupt, they shoot ash into the sky. 
 c. French settlers first came to Canada in 1604; the weather was cold and it was hard to grow food. 
 d. In 1845, Frederick Douglass published his autobiography; it became a bestseller.