Weakness in a CBC-like XOR cipher
Fundamental ideas: 

$ a \oplus a = 0$

$ a\oplus 0 =  a$

also if $K = B_{1}$ , $E_{1} = 0$ and as per your scheme $ B_{2} \oplus E_{1} = E_{2} = B_{2} $

As a result of you setting $K = B_{1}$  you cannot decrypt the first block if you do not know $B_{1}$. 
This system is fundamentally broken, it can never be used in practice. This assignment question sadly does not give the correct idea of CBC . Practical CBC mode of Block Ciphers like AES work in a very different way. 
For example if we have an Encryption function $E_{k}$ then this is how CBC encryption works. 

$C_{0} =E_{k}(IV\oplus M_{0})$

$C_{1} =E_{k}(C_{0}\oplus M_{1})$ 
and so on... 
As you can see there is an application of a very strong encryption function $E_{k}$ which could be something like AES. Here you are doing a simple XOR. 
Prima facie, your your scheme is subject to known - plaintext attacks and a 64 bit key is always brute force search able. I might have missed some crucial points but this is what I can think of now !!!