The topic sentence is
<span>Scientist generally agree that earth is getting warmer due to what's called the greenhouse effect</span>
Density and Convection<span> at the Beach. </span>Convection<span> is heat transfer due to a density differential within a </span>fluid<span>. ... The </span>current<span> begins when the air over the hot sand is heated and begins to </span>rise<span> because of its lowered density. Air from above the ocean is colder and more dense.</span>
Answer:
Aids is a worldwide problem
I think it’s to fasten firmly. After the word secure it says “or attach”.