system: You are an expert security code reviewer. You will be shown a source code file along with a multiple-choice question listing several function or class names from that file. Your task is to identify which one contains the specified CWE vulnerability.

OUTPUT FORMAT: Respond with the single letter of the correct option, wrapped in \boxed{...}. Example: `\boxed{B}`. Output the boxed letter ONLY -- no explanation, no other text.

user: ```python
#!/usr/bin/env python3
# Combine the data in "genotypes.txt", "markers.txt" and "families.txt"
# and convert them into a CRI-MAP .gen file
#
# This is a python version of the perl script discussed in my "Intro to perl"
# at http://www.biostat.wisc.edu/~kbroman/perlintro/index.html#ex2
#
# This is the python 3 version; see also convert2.py

def read_markers (filename):
  "Read an ordered list of marker names from a file."
  with open(filename, 'r') as f:
    lines = f.readlines()
  return [line.strip() for line in lines]

class Person:
  "Person class, to contain the data on a subject."
  def __init__ (self,family, id, dad, mom, sex):
    self.family = family
    self.id = id
    self.dad = dad
    self.mom = mom
    self.sex = "0" if sex == "2" else sex # convert from 1/2 to 1/0
    self.famid = family + '-' + id
    self.gen = {}

def read_families (filename):
  "Read family info and return a hash of people."
  with open(filename, 'r') as file:
    file.readline() # header row
    people = {}
    for line in file:
      vals = line.strip().split()    # strip removes leading and ending white space
      person = Person(vals[0], vals[1], vals[2], vals[3], vals[4])
      people[person.famid] = person
  return people

def parse_genotype (string):
  "Clean up string -> genotype"
  string = string.replace(' ', '')
  string = "0/0" if string == "" else string
  return string.replace('/', ' ')

def read_genotypes (filename, people):
  "Read genotype data, fill in genotypes within people hash"
  with open(filename, 'r') as file:

    header = file.readline().strip().split()
    header = header[1:] # omit the first field, "Marker"

    for line in file:
      marker = line[:9].replace(' ', '')
      line = line[9:]
      for i in range(len(header)):
        person = header[i]
        start = i*7
        people[person].gen[marker] = parse_genotype(line[start:(start+7)])

def get_families (people):
  "Return a vector of distinct families"
  return set([people[key].family for key in people])

def get_family_members (people, family):
  "Return a vector of famids for subjects within a family."
  return [key for key in people if people[key].family == family]

def write_genfile (filename, people, markers):
  "Write genotype data to a file, in CRI-MAP format."
  with open(filename, 'w') as file:

    families = sorted(get_families(people))
    print(len(families), file=file)

    print(len(markers), file=file)
    for marker in markers:
      print(marker, file=file)

    for family in families:
      print(family, file=file)
      members = sorted(get_family_members(people, family), key=lambda famid: int(people[famid].id))
      print(len(members), file=file)

      for famid in members:
        person = people[famid]
        print("%s %s %s %s" % (person.id, person.mom, person.dad, person.sex), file=file)

        for marker in markers:
          print(person.gen[marker], end=" ", file=file)
        print(file=file)


if __name__ == '__main__':
  # file names
  gfile = "genotypes.txt" # genotype data
  mfile = "markers.txt"   # list of markers, in order
  ffile = "families.txt"  # family information
  ofile = "data.gen"      # output file

  # read the data
  markers = read_markers(mfile)
  people = read_families(ffile)
  read_genotypes(gfile, people)

  # write the data
  write_genfile(ofile, people, markers)

```

Which of the following functions or classes contains a `CWE-312: Cleartext Storage of Sensitive Information` vulnerability?

  A) read_families
  B) write_genfile
  C) get_family_members
  D) Person
Answer: B