The two are potassium and rubidium. Why? Na is an alkali metal. As you go down the metals become more reactive. 
Calcium has 20 protons. Why? The atomic # is 20 (the one at the top). 
It has 20 neutrons. Why? The atomic mass (the number on the bottom) is ~40, ignoring electron mass. 40- 20 protons = 20 neutrons. 
It has 20 electrons. 
Why? The # of protons and electrons is the same. Ions aren't on the periodic table.
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Answer/Explanation:
Onions are stem modifications of bulbs. Onions grow during a cold weather, but forms bulbs when the weather is warm. Onions can either be grown using seeds or through their bulbs. That is why, the stem modification of onions are bulbs. Tuber is from potatoes, whereas corm is from crocuses and gladioli.
 
        
                    
             
        
        
        
The greenhouse effect.
Hope this helps!
-Payshence xoxo
        
             
        
        
        
Pluto is yo answer ma maaan
        
                    
             
        
        
        
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