1-Why !I couldn't get you on the phone this morning.
-we __tennis in the yard when you phoned me.
A must be playing B must have been playing
only must have been doing indicates what was happening in the past..
A is not correct in that it indicates only what is happening ,not what was happening
2-do you still remember when we went to the Great Wall?
-I can't remember it well,but __it have been sometime last May
a should b could
Here,the person who responded does not have any certainty with respect to the statement.Thus could is used,whereas should imples a certain degree of certainty