True or False with detailed and clear explanation 
11. Before substitution in a facultative obligation, only the principal obligation is due.
 
12. In a facultative obligation, only one obligation is due and demandable.
 
13. When there is multiplicity of p of parties in an obligation, the presumption is that the obligation is joint.
 
14. Anyone who claims that an obligation is solidary must prove it even as solidarity has been provided by law
 
15. The phrase, “joint and several” refers to a solidary obligation.
 
16. Solidarity must be clearly and positively expressed.
 
17. Solidarity may be stipulated upon by the parties to an obligation.
 
18. In a joint indivisible obligation, the object is joint while the liability is indivisible.
 
19. When the object of an obligation is a definite thing. the obligation is deemed Indivisible
 
20. No joint creditor can act in representation of the other joint creditors.