从A、B、C和D四个选项中，选出可以填入空白处的最佳选项，并在答题纸上将该选项标号涂黑。
1. Many Europeans explored _______ continent of Africa in the 19th century with the determination to discover _______ unknown.
A. /; the B. the; an C. /; an D. the; the
2. Clothing made of man-made fibers has certain advantage over ______ made of natural fibers like cotton, wool or silk.
A. this B. one C. that D. what
3. Francis Preston Blair, Jr., Union Army general during the Civil War, ________ born in Kentucky, lived and practiced law in Missouri.
A. as B. although C. as was D. although being
