Help with a question

[mngeow]

Ok for one of my school past year papers had this question,

"Section C Long Question (20 marks)

C1. A 2-bit multiplier is a combinational circuit that multiplies two sets of 2-bit binary numbers. The block diagram in figure C1a illustrates. Number B and Number A are two sets of 2-bit binary numbers which are applied at the inputs of the multiplier; the output of which is the product of B and A (i.e. B times A), a binary number of up to 4 bits

(a) Complete the truth table of this 2-bit multiplier circuit in your answer booklet using the format shown in Table C1. Four input combinations and their output values have been completed for you. (12 marks)

Number B Number A Multiplier Outputs (LSB)
B1 B0 A1 A0 Y3 Y2 Y1 Y0 Remarks
0 0 0 0 0 0 0 0 0 X 0 = 0
: : : : : : : :
0 1 1 0 0 0 1 0 1 X 2 = 2
0 1 1 1 0 0 1 1 1 X 3 = 3
: : : : : : : :
1 1 1 1 1 0 0 1 3 X 3 = 9
Table C1
(b) Using the 74151 multiplier IC (figure C1b) and one other logic gate, implement the 2-bit multiplier circuit for output Y1 only. You must label clearly, all inputs and output using the same variable names as those used in the block diagram and truth table. For consistency, assign the input variables A0 to S0, A1 to S1, B0 to S2, etc. (6 marks)

(c) How many 74151 ICs are required to implement the complete 2-bit multiplier circuit?
(2 marks)
"
The MSB's are A1,B1 and the LSB's are A0,B0

Here's the complete truth table for easier reference
"
Number B Number A Multiplier Outputs (LSB)
B1 B0 A1 A0 Y3 Y2 Y1 Y0
0 0 0 0 0 0 0 0
0 0 0 1 0 0 0 0
0 0 1 0 0 0 0 0
0 0 1 1 0 0 0 0
0 1 0 0 0 0 0 0
0 1 0 1 0 0 0 1
0 1 1 0 0 0 1 0
0 1 1 1 0 0 1 1
1 0 0 0 0 0 0 0
1 0 0 1 0 0 1 0
1 0 1 0 0 1 0 0
1 0 1 1 0 1 1 0
1 1 0 0 0 0 0 0
1 1 0 1 0 0 1 1
1 1 1 0 0 1 1 0
1 1 1 1 1 0 0 1
"
I can get the truth table but I don't know how to implement the outputs for Y1 onto the MUX.