Why is there more than one single locus probe used in actual paternity dna test?

There is the application of more than one single locus probe in the actual paternity test as one receives half of the genetic substances from the mother and half from the father. There is more than one probe that is needed to examine both parent's identity.  

Or it can be said that the locus probes are utilized to determine various intervening repetitive sequences of DNA that are situated in different chromosomes and leads to the production of 30 and 20 bands, thus, one can avoid the chances of a similar pattern due to multi-band patterns.  

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.