If each washer has a mass of 4.9 g, then the mass of two washers in kilograms would be . Given that F = ma, and if 4 washers are attached to the string and car instead of 2, then the applied force on the car will be .

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

where are the choice options?

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.