This can be done by using the fundamental principle of counting.

There are altogether 5 slots for solving the problem.

In the first slot we have 1 because we have to attempt part A first.

In the second slot we have 3 because there are 3 options for first part.

After one of the option is chosen the third slot will contain 2 option.

After first part is complete there remains two slot.

These two slots will contain number 4 and 3 respectively because for second part there are 4 options and two must be done.

Therefore ans = 1*3*2*4*3 = 72

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This is my response to the question and may be incorrect. Feel free to rectify any mistakes