Read the following passage about seawater treatment and mark the letter A, B, C or D on your answer sheet to indicate the option that best fits each of the numbered blanks from 18 to 22.

Why don't we get our drinking water from the ocean?
If you're stuck at sea and thirsty, drinking seawater might seem like an option, but it's dangerous, Seawater is much saltier than your blood, (18), making you more dehydrated. Your kidneys try to get rid of the extra salt, but they need fresh water to do that. Without enough fresh water, drinking seawater can lead to salt poisoning, causing serious damage to your cells and organs.
To survive in this situations, you need to find or create a source of freshwater. In the 4th century BCE, a philosopher described two ways of turning seawater into drinkable water: thermal desalination and reverse osmosis. Thermal desalination, an ancient method, (19) _______. Reverse osmosis uses pressure to push seawater through a membrane that filters out the salt. Both methods are still used today to make seawater drinkable, though each has its challenges. In terms of their efficiency, reverse osmosis is more energy-efficient and less wasteful than thermal desalination. (20) _______.
An alternative source of freshwater is wastewater. Reverse osmosis can purify wastewater more easily than seawater, as it contains less salt and fewer contaminants. By producing safe, drinkable water with reduced energy consumption, (21) _______.
In a survival situation at sea, drinking your urine or eating protein-rich foods is not a good idea, as it can make you more dehydrated. (22). In extreme cases, people have resorted to drinking blood from animals as an emergency hydration source, though this is far from ideal and should only be used if necessary.
(Adapted from https://www.ted.com)