Answer:
A & D
Explanation:
A single-displacement reaction is a chemical reaction whereby one element is substituted for another one in a compound and thereby generating a new element and also a new compound as products.
From the options, only options A & D fits this definition of single-displacement reactions.
For option D: Both left and hand and right hand sides each have one element and one compound. We can see that K is substituted from KBr to join Cl to form KCl and Br2 on the right hand side.
For option A: Both left and hand and right hand sides each have one element and one compound. We can see that OH is substituted from 2H2O to join Mg to form Mg(OH)2 and H2 on the right hand side.
The other options are not correct because they don't involve only and element and a compound on each side of the reaction. 
 
        
                    
             
        
        
        
The answer is <span>It protects the seed until it matures.
Through the process of elimination:
- </span><span>It protects the seed until it matures. YES. A female pine cone the unfertilized seeds until they become mature. Then, the cone opens and seeds fall out of the cone.
</span><span>- It makes food for the adult plant. NO. The food of plants in made of green leaves that contain chlorophyll in the process of photosynthesis.
- It allows for photosynthesis to occur. NO. The cones have nothing to do with photosynthesis but they are responsible for reproduction.
- It acts as an object to attract birds. NO. The birds look for colourful flowers while cones are woody.
- It serves as an ornamental object on the tree. NO. Every part of plants has more important function than serving as an ornamental object.</span><span>
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Answer:
The relative velocity of an object A with respect to another object B.
Explanation:
The relative velocity of an object A with respect to another object B is the velocity that object A would appear to have to an observer situated on object B moving along with it.
 
        
             
        
        
        
Answer:
A plain mirrior is a mirrior with flat reflective surface.
hope it is helpful for you.