Can you please explain what the objective is?
        
                    
             
        
        
        
An estuary is where salt water and fresh water meet. Estuaries are usually found at the edge of the coast, where the ocean water, or salty water, meets river, lake, any fresh water source, water.
A tributary is something completely different. It is a, usually smaller than the main river or lake, river/stream stemming off a bigger river or lake, or flowing into that river or lake.
        
             
        
        
        
Answer:
Option d) Gulf stream
Explanation:
The Gulf stream along with the drift in North Atlantic  which is the northern extension of the Gulf stream is a warm current of Atlantic ocean.
It has its origin in the Gulf of Mexico and extends upto the tip of Florida along the coastlines of the eastern part of the NewfoundLand and the U.S before passing through the Atlantic Ocean.
These are responsible for the milder winter climates of Northern Europe and the U.S 
 
        
             
        
        
        
Flammable gas, consisting largely of methane and other hydrocarbons, occurring naturally underground Advantages<span> & Disadvantages of </span>Natural Gas<span>. </span>Natural gas <span>burns cleaner than other fossil fuels, producing half the carbon dioxide as coal and about a third less than oil. It also emits fewer amounts of toxic chemicals like nitrogen oxides and sulfur dioxide.</span>