a.rb

class A
  @@a = 1
  @a = 2
  a = 3
end

exercise_four.rb

last_a = nil
tracer = lambda { |event, _, _, _, bind, _|
  value = (eval("a", bind) rescue nil)
  last_a = value if value
}
set_trace_func(tracer)

load "./a.rb"

arr = []

arr << A.class_variable_get(:@@a)
arr << A.instance_variable_get(:@a) 
arr << last_a

puts arr