Python Collections. Dictionaries. Teacher Stats Challenge. Task 4. The correct solution does not pass

# The dictionary will look something like:
# {'Andrew Chalkley': ['jQuery Basics', 'Node.js Basics'],
#  'Kenneth Love': ['Python Basics', 'Python Collections']}
#
# Each key will be a Teacher and the value will be a list of courses.
#
# Your code goes below here.
def num_teachers(d):
    ctn=0
    for i in d:
        ctn+=1
    return ctn

def num_courses(d):
    ctn=0
    for i in d:
        ctn+=len(d[i])
    return ctn

def courses(d):
    l=[]
    for i in d:
        l=d[i]
    return l


def most_courses(d):
    span = 0
    for i in d:
        if len(d[i]) > span:
            span = len(d[i])
            s = i
    return s