Hi in this question

A typical intravenous dose of I-125 for a thyroid diagnostic test is 100 u Ci. What is this dosage in megabecquerels (MBq)? (3.7 x 1010 Bq = 1 Ci)
my book solved it like this:
100 u Ci * 1Ci/1x10^6uCi * 3.7x10^10/1Ci * 1MBg/1 x 10^6 Bq = 3.7 MBq. My question is where did the 1 x 10^6 u Ci come from?

exactly what does u mean? 1E-6. so the 1E6 times that makes the second term

(1Ci/1E6uCi) a unit factor for conversion.