Answer:
Let's say 6 statements are lies.
A's 1st statement and C's 1st statement negate each other. One of them is a lie.
C's and
D's negate each other. One of them is lying. Therefore, there are 4 additional false statements.
Assume A is guilty. If so, A's second statement, B's second statement, and
D's first statement are the additional lies.
Assume
D is guilty. If so, A's second statement, B's first statement, and
D's third statement are lies. This also only makes five false statements.
D did not do it.
Assume C did it. If so, A's second statement,
D's first and third statements are false. This again, makes only five false statements.
After ruling out suspects A, C and
D, B is the culprit. B's third statement, C's second statement, and
D's first and third statements are the additional false statements. This adds up to six.