Problem $1.$ If s is a string object, s$.$replace$($u$,$ v$)$ returns a string with all occurrences of u in s replaced with v$,$ and s$.$count$($u$)$ returns the number of occurrences of u in s$.$ For example, "abba".replace$("$b$","$a$")$ returns "aaaa", "abba".count$("$b$")$ returns $2,$ and "abba".count$("$z$")$ returns $0.$ Consider the following functions:

a$.$ What does f$("$abracadabra$","$a$")$ return?

A $"$ br c d br $"$

B $"$abrcdbr$"$

C $"$brcdbr$"$

D $"$bracadbr$"$

E $"$abracadabra$"$

b$.$ What does len$($f$("$abracadabra$","$b$"))$ return?