The correct answer is option 1: 1 only. When a tropical cyclone moves to land, much of its severity is reduced because its source of moisture is cut off. Tropical cyclones derive their energy from warm ocean waters, which provide heat and moisture for their development and intensification. When a cyclone moves over land, it loses this source of moisture, causing it to weaken.

Option 2: The friction over land does not kill tropical cyclones. While friction can cause some weakening of the cyclone, it is not the primary factor in reducing their severity. The main reason for the weakening of tropical cyclones over land is the loss of their moisture source.

Therefore, the correct explanation is that when a tropical cyclone moves to land, much of its severity is reduced because its source of moisture is cut off.