def dna_match(long_sequence, short_sequence):
    """
    Find the part of long_sequence which is most similar with short_sequence.
    :param long_sequence: string, the combination of A, T, C, G.
    :param short_sequence: string, the combination of A, T, C, G.
    :return ans: string, the most similar part.
    """
    most_similar = 0
    place = 0
    for i in range(len(long_sequence) - len(short_sequence) + 1):
        count = 0
        for j in range(len(short_sequence)):
            # Record it if the dna is the same.
            if long_sequence[i+j] == short_sequence[j]:
                count += 1
        # Find most similar part and record its place.
        if count > most_similar:
            most_similar = count
            place = i
    ans = long_sequence[place: place+len(short_sequence)]
    return ans