What did scientists observe in ocean floor which indicates that the Pacific Ocean is becoming smaller?

A. Plates floating on the mantle.
B. Spreading.
C. Continental drift.
D. Subduction.

B. Spreading.

The correct answer is B. Spreading.

Scientists have observed evidence of spreading at the ocean floor, which indicates that the Pacific Ocean is becoming smaller. This observation is based on the concept of plate tectonics, which explains the movement of the Earth's lithospheric plates.

To understand how scientists arrived at the conclusion that the Pacific Ocean is becoming smaller, let's break down the options:

A. Plates floating on the mantle: This refers to the concept of the lithospheric plates, which float on the semi-fluid layer of the Earth's upper mantle. While this is an accurate description of plate behavior, it doesn't specifically indicate that the Pacific Ocean is becoming smaller.

B. Spreading: This refers to the process of seafloor spreading or the creation of new oceanic crust at the mid-ocean ridges. As new crust forms, it pushes apart the existing crust on both sides, causing the ocean basin to expand. In the case of the Pacific Ocean, seafloor spreading at the East Pacific Rise and other mid-ocean ridges indicates that the ocean is indeed expanding.

C. Continental drift: This term refers to the movement of the Earth's continents over geological time scales. While continental drift is associated with plate tectonics, it doesn't directly indicate that the Pacific Ocean is becoming smaller.

D. Subduction: This refers to the process where one tectonic plate slides beneath another plate into the Earth's mantle. While subduction occurs at convergent plate boundaries, where plates collide, in the case of the Pacific Ocean, subduction is more relevant to the western boundary. It doesn't provide direct evidence that the entire Pacific Ocean is becoming smaller.

Therefore, the correct answer is B. Spreading. The observation of seafloor spreading provides evidence that the Pacific Ocean is expanding rather than becoming smaller.