Speech is a<em> very important communication tool which allows a person to interact with his audience.</em> It can be used in different settings, according to its intended purpose. It primarily has three purposes:<em> to inform, persuade and entertain people.</em>
An example of a speech that is intended to inform is a topic related to societal issues these days, such as "Global Warming." The speaker may set his intention to educate people or to provide awareness on the different reasons why global warming is becoming widely spread in the world. So, in this case, his goal is to let the audience know about something they haven't heard about.
Is - two billion dollars is a unit, one unit so it is singular and you use the singular verb think - scientists is a plural, therefore, you must use the plural verb form of think
