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Explanation:

x1x2x3x4x5x6xcan be of the form abcdabc or abcabcd.

abc can be chosen in 10 × 10 × 10  = 1000 ways.

d can be chosen in 10 ways.

∴ x1x2x3x4x5x6xcan be chosen in 2 × 1000 × 10 = 20000 ways.

However, this includes the ways in which all of abc and d are 0, 1, 2, 3, …, 9, twice.

We subtract these numbers to get 20000 – 10 = 19990

We also don’t want 2000000.

∴ The total number of ways = 19989.

Hence, option (c).

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