Step 1: Total earth surface.
The earth's total surface area is about 510 million square kilometers.
Step 2: Oceans and land distribution.
- About 71% of the surface is covered by oceans (Pacific, Atlantic, Indian, Arctic, Southern).
- The remaining 29% is land (continents and islands).
Step 3: Eliminate other options.
65%, 79%, and 81% are incorrect figures. The scientifically accurate figure is 71%.
Final Answer: \[ \boxed{\text{Oceans cover 71% of earth's surface.}} \]
Fill in the blank with the correct option.
The teacher believed that the student’s sudden lack of interest in class was an ..........., as he had always been enthusiastic and attentive.