In this task, you need to answer the given multiple-choice question on the physics. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

[Q]: Problem: a hollow iron pipe is 21 cm long and its external diameter is 8 cm . if the thickness of the pipe is 1 cm and iron weights 8 g / cm cube , then find the weight of the pipe .
Options: ['a ) 3.696 kg', 'b ) 3.686 kg', 'c ) 2.696 kg', 'd ) 2.686 kg', 'e ) none of these']
[A]: a

[Q]: Problem: what is the maximum number of pieces of birthday cake of size 5 " by 5 " that can be cut from a cake 15 " by 15 " ?
Options: a ) 5 , b ) 10 , c ) 16 , d ) 20 , e ) 9
[A]: e

[Q]: Problem: matt and peter can do together a piece of work in 20 days . after they have worked together for 10 days matt stops and peter completes the remaining work in 10 days . in how many days peter complete the work separately .
Options: a ) 21 , b ) 20 , c ) 22 , d ) 23 , e ) 24
[A]:
b