It is dangerous to clean well remaining
closed for a long time, why?

Because of the accumulation of high percentage of carbon dioxide when a person enters into the closed well for a long time it becomes harder for the person to breathe in and hence may lead to death.

hope It helps

Because of the accumulation of high percentage of carbon dioxide when a person enters into the closed well for a long time it becomes harder for the person to breathe in and hence may lead to death.

they are both synthetic elements

explanation:

d

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.