Passage: With Tiberius' departure, succession rested solely on Augustus' two young grandsons, Lucius and Gaius Caesar. The situation became more precarious in AD 2 with the death of Lucius. Augustus, with perhaps some pressure from Livia, allowed Tiberius to return to Rome as a private citizen and nothing more. In AD 4, Gaius was killed in Armenia, and Augustus had no other choice but to turn to Tiberius.The death of Gaius in AD 4 initiated a flurry of activity in the household of Augustus. Tiberius was adopted as full son and heir, and in turn he was required to adopt his nephew Germanicus, the son of his brother Drusus and Augustus' niece Antonia Minor. Along with his adoption, Tiberius received tribunician power as well as a share of Augustus' maius imperium, something that even Marcus Agrippa may never have had.In AD 7, Agrippa Postumus, a younger brother of Gaius and Lucius, was disowned by Augustus and banished to the island of Pianosa, to live in solitary confinement. Thus, when in AD 13, the powers held by Tiberius were made equal, rather than second, to Augustus' own powers, he was for all intents and purposes a "co-Princeps" with Augustus, and, in the event of the latter's passing, would simply continue to rule without an interregnum or possible upheaval.However, according to Suetonius, after a two-year stint in Germania, which lasted from 10−12 AD, "Tiberius returned and celebrated the triumph which he had postponed, accompanied also by his generals, for whom he had obtained the triumphal regalia. And before turning to enter the Capitol, he dismounted from his chariot and fell at the knees of his father, who was presiding over the ceremonies.” "Since the consuls caused a law to be passed soon after this that he should govern the provinces jointly with Augustus and hold the census with him, he set out for Illyricum on the conclusion of the lustral ceremonies."Thus, according to Suetonius, these ceremonies and the declaration of his "co-Princeps" took place in the year 12 AD, after Tiberius' return from...

Question: Who was the grandfather of Lucius and Gaius Caesar?
Augustus