Find a phrase-structure grammar for each of these languages (more than one answer possible).
a) L = aⁿ bⁿ mid n geq 0
b) L = aⁿ bⁿ cⁿ mid n geq 0
c) L = aⁿ bⁿ cⁿ dⁿ mid n geq 0
d) L = aⁿ bⁿ dⁿ mid n geq 0