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From Wikipedia - In general Triple DES with three independent keys (keying option 1) has a key length of 168 bits (three 56-bit DES keys), but due to the meet-in-the-middle attack the effective security it provides is only 112 bits.

So 3Des would be wrong because it doesn't use a _single_ 56 bit key but it uses three of them.

Your answer would still be wrong as the effective key strength is 112 bits.